hmmm. Well first the only right answer is nothing and everything. If you buy it at a deep enough discount and you have to replace everything then you have no risk if you buy it at fair market value then everything is a risk if you miss it. With that said. The costly items like a roof or house systems are actually pretty easy to spot if you know what to look for. In the attic if you see discoloration on the underside of the roof it may have a leak. If it raining and nothings coming in your ok. If you smell a damp musty smell there is a moisture issue. Not always alarming but something to remedy. Check the date on the furnace see to see the age. Open the front panel and check filter to see how it's maintained.
The biggest thing I've ran into was a serious roach infestation. Along with mice termites and bed bugs. Those can be killers because you might think it's ok and overpay. Check the kitchen cabinets. Inside ,above , below !!!