Heres my proposed best build according to parameters you mentioned:

• 1 kidney

• 1/3 liver

• 1 arm

• 1 leg

That should cover the expenses for 5090 if you get a good deal. You dont need other parts nor monior or kb/mouse, just diy it with some soldering iron & YT tutourials. For case, just use old cardboard box, prefferably piss soaked