There's a lot of information from your friendly federal government online that's a good starting point. First, I'd find out whether any part (or all) of the land is in a flood zone. If so, flood insurance would be required to borrow any money to build (unless you have a survey done and establish that any structure are not in the flood plain). That will also provide you with a rough idea of the chances of flooding. Second, the USDA has soil maps that cover almost the entire country. That's a free way to get a rough idea of the soils you're dealing with.
After that, most land grant universities will do soil tests at a reasonable cost. Also check whether there are restrictions on wells--this varies by state but it's useful to know. Also check the tax situation and what exemptions are available. That's usually done by the county appraisal district, but states vary. Finally, make sure you don't have any access problems. In some states easements for access isn't guaranteed and even putting a driveway in may require a permit.