Charlie figure out what you can afford to do first that will not have to be redone later. An example( Fix the roof now so it does not leak on the new drywall under it.) Fix the roof, windows and make sure you have good doors. The inside walls can be done later as far as finishing them. You do need a bathroom with privacy along with the bed rooms you and your parents use , but the rest can wait. The sheet rock can go up and be taped later except for the kitchen because of grease. If you need to make the bathroom larger now is the time. Older parents need more room for walkers and such . We are going through this with my dad now. Off my soap box now. Good Luck Charlie. Don't forget the vent for your paint booth.