You can also look at thier other world - DragonLance.
If you look at the Knights of Paladine (Solamnic Knights), and later when the Knights of Takhisis came to be.
The two orders, LG and LE had to come together into one to fight a bigger threat (during the fifth age).
So the concept a LG Paladin can't/won't work with other alignments is not fully true.
Sure, when it came out, I always thought of them as being like a Knight of the Round Table/Camalot/King Arthur, displaying the best of the best, showing others how one should be.
So with regards to wealth, since such "knights" had to come from a wealthy "family" to begin with (to be allowed to be trained as a squire then into the knighthood as a knight) the family had to be about to afford this.
Other "knights" (like in DragonLance), they could own land and build castles, what the rules say a "modest" or "small castle" is a grey area. Modest/small as compared to what?
Lord Soth's castle was HUGE!
The Knight's and their fortresses/castles had to be paid for as well. Is that part of getting rid of the individual's excessive wealth, but for the benefit of the knight's order? The order is wealthy while the individual remains "modest"?