You seem to be under the impression that the budget system is there as an actual monetary limitation xD No, it's to keep you from breaking the game. It's an actual show of how much the game can safely handle. Ships like that would never be possible.
indestructible vehicles only makes the vehicle indestructible. Making the spartan invincible is the only way to give him/her indestructibility. Oddly enough, player traits carry over to vehicle traits as well, so their damage resistance is reflected by the vehicle, and their damage setting is the splatter damage setting O.o
The DMR is more accurate, but the BR's burst makes that last headshot easier, meaning it's better at close range, when missing that 5th shot could mean the difference between life and death. The DMR is better at longer distances, simply because of the accuracy. The carbine is just a fail.
On Reach, you could simply spawn ghosts and set it to UNSC only, but I tried that on here, and they simply didn't spawn, so I guess there isn't a way to do it. Either that, or I have it on hogs only and I need to go test that.