Does Spelljammer have different technology levels?
Yep. Gunpowder is common (especially with the Giff) and laser weapons (and probably antimatter weapons) are present (based on the Gnome Ceremorphs, at least). And, you know, space travel is common.
Spelljamming Helms, Gadabouts (living plant space suits with butterfly wings that the elves use), exotic materials from Wildspace, equipment from all across the multiverse, and so on. You could have a Pallid Elf Graviturgist (from Exandria) with a Yklwa (from Chult), a Gadabout (from wherever the Elven Imperial Navy gets theirs), and a laser rifle (from the Mind Flayers) that rules over a giant floating city powered by a Netherese Mythallar.
Basically all equipment from across all of D&D could appear in the setting.
They have stated that the upcoming Spelljammer books will have new spells in them, so yes. Classically, there were spells to create/locate portals that could take you through a Crystal Sphere, Create/Destroy Air, and spells to allow a Cleric to contact their god outside of their home Crystal Sphere. I'm pretty sure most of these won't make it to 5e (Create/Destroy Air might), but 5e Spelljammer will have some unique spells.
For my 5e Spelljammer campaign, I always thought Dunamancy (from Explorer's Guide to Wildemount) fit pretty well with the setting.
In essence is a Spelljammer physically different from any other ship when you take the Spelljammer helm away?
Uh, yeah. Most of them are quite different. Gnomish ships are steampunk/gearpunk mechanical nightmares, Mind Flayer ships typically look like a
nautilus or cuttlefish, Neogi ships look like spiders, Dwarven "ships" are just flying mountains that look like a decapitated dwarf head, and elven ships are just giant living plants. There are also whale-shaped ships, manta ray-shaped ships, shark-shaped ships, wasp-shaped ships, and so on.