Roof.

This one is important but may need to be farmed out to a pro. However on a single story home with a low slope, and good ladder,shoes ,balance and insurance if you are in decent shape you can do this yourself. You are looking primarily at the overall condition such as age and weathering of the roofing material. You also will be closely inspecting where any two rooflines come together. Where any wood comes in contact with the roof you are looking for rot, peeling paint, etc. These areas are often the most neglected due to the inaccessibility of their location but they often take the most abuse from the weather due to their exposure. If you have any doubts at all about the safety of doing this yourself hire a professional home inspector. They know what they are looking for and will often cut a break on the price if all they have to do is the exterior of the roof. However a good one will often insist on doing an interior inspection of the attic to determine the soundness of the inside structure.