Soil type, adjacent land and what they have on them (spraying chemicals or not).

What's more, there's a garage with a basement underneath it.

Quads on a septic system is going to be a major expense and you should expect to pump it out every 2 years or so, so it needs a man hole cover.If it is going to be septic be SURE you have sufficient soil quality for your drain field, you will want lots of sand.

It might be a good idea to tie the site up as long as you can to get some soil testing and at least some preliminary drawings done by the civil engineer to see exactly what the site work costs will be. 

The Soil Connect app might be helpful.

Trade secret: Block and pier is an acceptable foundation for many permenant foundations.in Charleston SC all our foundations are block and peir as they are raised 7 to 12 feet to allow flood waters to flow underneath..   

Have your plumbers look underneath the tub for water, or mold.

Anybody (me or a later buyer) who built on the lot would risk losing it if a claimant won owner to the land underneath. 

"The brighter the dirt the better the soil"6.

Have done any soil testing or build studies so that you can transfer the land in a more opportune way than when you bought it?