The difference in build cost could add or subtract sqft but more of a quick estimate 

They will run your credit anyway later, but the bank can give you some estimates with the credit score.

There are a few items I have no idea about, I'm hoping someone here has some experience in the field and can provide a ball park means of estimate 3 costs:1.

Rehab costs that are under estimated by very wide margins.  

I'm just north of the lake, thinking of doing the same, have about the same equity, and my estimates ($3500/mo) seem to cashflow a couple hundred a month.

you have someone with very little means borrowing your 5k Em ( which is a rare EM in most markets) and even if they get it back chances are you not going to get them to pay U a dime of interest they will just go to the next gater lender.

Without a contract in place, and with no prices being quoted, or estimates made beforehand, we have what’s essentially an after the fact negotiation.  

There is a chance their unit is also pretty dated compared to the others or others in the same neighborhood.

I got several estimates from roofers and the replacement cost is around 13k (it's a ~1600 sqft house).

There’s a 99% chance you’re going to ask to much and sit empty for an extra month or two.