I'm looking for mechanical constructs to do tasks...
Considering that constructs to do tasks are generally beyond 21st century technology, and yet are simple enough exercises of magical power that anyone capable of casting 'unseen servant' can best the most modern robot technology, I fear that what you are looking for simply may not exist. Mechs for example aren't practical weapon systems now, and arguably never will be. Powered battle armor might come into its own at some point, but steam engines and the like just simply don't have enough energy storage for the weight. Golems are far more potent than any clockpunk robot you'd care to create without magic, and readily available within a 'high magic' setting. Magic is simply better.
Fire arms of any technology level in D20 are best handled as special cases of Ken Hood's firearms rules. He has the most comprehensive and effective rules for firearms I've seen anywhere.
If you have any questions about leveraging them to handle a particular class of early firearms, let me know and I'll help you out.
and clockwork/steam type stuff.
In general, clockwork automata realistically just doesn't work without liberal application of magic. Even with the best technology, mechanical calculators are huge and only function for short periods without breaking down in some fashion. It's not remotely possible to fit enough calculating power into a machine using gears to simulate any sort of complex behavior - there just won't be enough bytes of memory, and the more you add, the shorter amount of time before it enters a failure mode.
However, an accurate mechanical clock itself is a huge advancement in technology, allowing not only convenience and greater organization, but significant improvements in navigation.
things like mechanized lifts and maybe even carts/etc for traveling, and early on probably not much else battle-wise. Basically, stuff that will affect life outside of combat earlier on, but super low-tech.
Mechanically, much of this is available even in the middle ages and presumably many of the advancements - like the Otis safety elevator - could have been made if these things were in wide enough use that it had become a problem of widespread consideration using only low tech construction and craftsmanship. For example, the medieval and early moderns had elevators, automated trip hammers, saws, mills, grinders and a variety of other machines, powered pumps, and so forth. The big difference is that they relied on water and wind power (or occasionally animals), which greatly limited the amount of available energy and the locations you could make use if it.
It's worth noting that this sort of stuff already exists in default D&D, though it usually shows up and interacts with the PC's in the form of a trap filled tomb rather than a working factory complex. But any society mechanically complex enough to produce the astounding range of amazing mechanical traps seen in D&D, presumably has all the mechanical tools the medieval had and then some.
The big advance in the real world was the steam engine which let you hook up all those things to a small comparatively portable power source. If you have steam engines, you can have pretty much anything up to about 1900s technology - locomotives, steam powered cars, ironclads, automated looms, etc. Most of this you don't really need rules for unless you are doing some sort of economics subgame with the players as merchant industrialists, in which case, rules for technology are the least of your problems.
The first problem is again, magic is just better. Most D&D magic outperforms not only present day technology, but any conceivable near future technology in many regards. Long before technology could overtake it as an independent force, industrialized magic as technology would beat it hands down in all regards. D&D magic can with only a slight amount of repurposing relatively easily make household robots, full stealth chameleon suits, personal flying belts, forcefields, man portable artillery, ray guns, perpetual motion machines, and any number of other things. Cure disease and cure light wounds are light years better medical technology than exists now or at any point in the conceivable future. The main reason we don't do magic as technology to create a D&D that is filled with science fiction or science fantasy tropes is that we don't want to have that outcome and so arbitrarily set extra limitations on the magic when it comes up.
The second problem is that if you base your world on magic, the players aren't experts in how it works and it doesn't have to be completely coherent. But if you introduce basic technology, the players are probably experts in how it works and can make some pretty simple suggestions for improving the technology that in the real world took people 5-10 years to work out. Doing that repeatedly allows the players to accelerate the pace of technological improvement from new generations of technology every few game decades, to new generations of technology every few game weeks. You'll literally risking a 'Connecticut Yankee in King Author's Court' problem. In the real world, the main thing that separated the medieval Hand Gonne from say a Kentucky Flintlock rifle, was lots of trial and error. If you eliminate the trial and error, you've got a problem however you address it. If you say fiat, "No, you aren't allowed to do that.", the player may well feel you are unreasonably constraining player free will. If you on the other hand say, "That's player knowledge and not character knowledge, you need to do research and pass difficult skill checks in order to advance technology", then expect players to easily overcome whatever hurdle you put in front of them via min/maxing.
In my own game, I make the assumption that anything the players have the imagination to attempt, has been tried literally hundreds or thousands of times by NPCs over the course of the world's 5000 year unbroken written history. If players want to invent firearms, they can - though knowledgeable craftsman will advice them that they are largely wasting their time. If they persisted anyway, they'd find that the world contains no stable explosives. The world's chemistry is after all based on 4 elements - earth, fire, air, and water - and not the familiar periodic table. Gunpowder - even if the player's precisely describe the process for making it - simply doesn't work. Alchemy can get you explosives, but none that are stable in the long term and shock resistant. The best available explosives are roughly the equivalent of raw nitroglycerin in stability and gunpowder in terms of blast strength, even after you mix them with binders like chalk or sawdust. Attempting to equip an army with explosives in a world where relatively low level casters can produce magical fire or waves of force and where even a single trooper getting killed and dropping his weapon can set off a chain reaction is like sending an army of troops with suicide belts against a force holding the remote detonators. It's been tried a couple of times in history - once by goblins and a couple of times by mad men - and it always fails spectacularly. If guns worked in this world, they'd already have been invented.
Now of course, I could allow guns into my game if I wanted to. But by my calculations, a 1st level hobgoblin fighter with modern weaponry is about CR 6 and has some relatively nice features that keep him threatening until relatively high level. The problem with technology is that it mostly tends to level the playing field. It's not an accident that most romantic fiction is based off eras when defensive technology considerably outstripped offensive technology, leading to a situation where a single armored warrior could defeat a couple dozen foes with less advanced armor and weapons. The era of firearms - at least the era where both sides have firearms - is an era where offensive might has vastly outstripped defensive technology, leading to a situation where the mightiest and most experienced warrior can be easily killed by a mook. Consider for example the narrative role of the musket in Kurosawa's 'Seven Samurai'.