You're asking this board how to spend your money. Should run at least 3 pages.
The plane may or may not have "the minimum". Just bake the costs of getting up to the minimum into your asking price.
Minimum for VFR: Make sure your main 6 pack, transponder, and radios are in good working order. If it has a non ADSB transponder / if you don't have ADSB in as well as out, I would toss the transponder and get one with ADSB In and Out, like a Garmin 345. Sell off what ever you took out. Is your turn coordinator making funny scratching noises? Does your altimeter stick and bounce? Does your heading indicator precess after only 5 minutes? Address those issues first.
For IFR - that's a healthy list just by itself - A GPS navigator (like a GTN 650), dual CDI's, dual nav coms, and a lot of other options (like G275's, etc.) Others more experience in IFR will jump in here as well.