Are you paying the asking price for the house and the lot?
What is the flood zone of the lot?
When you say "we" estimated moving and setup costs, who is we? Was a house moving company involved in that estimate?
$130-150k seems like a wide range? What are similar aged/style 1,000 sq ft houses selling for in the area of the lot? That will help you narrow down your ARV.
To your questions:
1. Somebody in the transaction needs to know and understand real estate contracts to get those things right if you won't involve a realtor.
2. When you get your contracts with each seller (house and lot) the title attorney will do the searches for liens and other title-related concerns, not you.
3. A lot. Every usual concern and then a whole set of house moving related concerns. A very competent house moving pro, Ed Smith, Devillier House Movers, can advise you about those concerns. You can find him with an online search. My post was blocked when I included his phone number. :-) NOTE: I have no business affiliation with this person/company.