Perform due diligence, including soil tests, environmental assessments, and surveys to ensure the land is suitable for construction.

Often, the land UNDERNEATH power lines have easements on them, that go five or ten feet in both directions, sometimes more, which means you can't build anything. 

All of that concrete rubbish has to be hauled away and new soil brought in.

MAKING SURE THE SOIL IS TESTED TO MAKE SURE IT CAN STAND THE PRESSURE I ASSUME, LAYING THE SLAB OF CONCRETE KEEPING IN MIND OF UTILITIES AND ETC.

If you intend to hook it up it would be smart to make sure it is not blocked either by tree roots or crushed at some point underneath the lawn or someone poured cement down it or something like that.

Is market like Texas where they often have soil/foundation issues, or like MN/the Midwest where the asphalt shingles might have hail damage?

What's the cost to build - one might be more advantageous because of terrain or soils (ask the sellers for a soils report). 

At this point, this tenant sounds like they're all bark with very little bite - I'd just keep documenting everything in writing (as it sounds like you've been doing) and sending everything to them via email as a backup as well, and only involve the lawyers if the need actually arises (e.g. if they file a rent escrow action; if you need to start eviction process for non-/partial payment; if you get them to agree to a mutual lease termination; etc).If you do end up going with the carpet replacement option, I would also recommend going one step further: in addition to replacing the carpet and the padding underneath, also take that opportunity to put down a coat or two of heavy duty Killz on the subfloor before the new carpet gets installed. 

When you guys throw down the cheep carpet do you also put padding underneath it?

.- The unattached garage (built into same hill) has drain run off that goes underneath the driveway and into the road.