Two of the most important things to check are the roof and the foundation. Then you have the wiring and plumbing to think about. Your best bet would probably be to hire a home inspector or a professional carpenter to take a look at it. It will cost you some money but it could save you a lot more if you miss a major problem.

a leaking roof first.
mold in the attic.
water in the basement as well as mold.
the quality of the electrical system, wires, receptacles,light fixtures.
the plumbing including a septic tank if that's what it has.
those would be most important to me.
friends in the business might be able to give you help or advise as to what you need to do.
above all. check the roof first.

lead paint/ check with your local goverment they should be offering classes.
Mildew/ bleach
Water and gas leaks
leaky or inoperatable furnace and hot water heater

Adam/Penny has the right idea. In general, moisture problems usually cause the most trouble for a house. You should also assume that critters have taken up residence, particularly in the attic or crawlspace or under decks; if you see signs of animal activity you can call an exterminator. A home inspector can assess the place but it sounds like you and your friend might have the skills to catch a lot of potential problems. Here's a checklist of things to look for:
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some of the most important things you need to do: check the roofing, wiring, foundation, the heating and cooling units, and plumbing