You don't need pre-approval or anything that formal but at least know that they will refi the property given an appraisal.For further research, read up on the BRRRR strategy (Buy Rehab Rent Refi Repeat).

In the absence of formal or informal lease terms (and considering the lack of rent), the sons are probably not tenants and thus eviction may be an inappropriate solution. 

Keep in mind that, like a "no-point" loan, the interest rate will be higher than if you obtain a loan that has costs associated with it.noteA legal document that obligates a borrower to repay a mortgage loan at a stated interest rate during a specified period of time.note rateThe interest rate stated on a mortgage note.notice of defaultA formal written notice to a borrower that a default has occurred and that legal action may be taken.original principal balanceThe total amount of principal owed on a mortgage before any payments are made.origination feeOn a government loan the loan origination fee is one percent of the loan amount, but additional points may be charged which are called "discount points."

Tell them you will pay "appraised value" then hire a formal appraisal.

You can clearly spell out, in the articles, who owns what share of the company, how proceeds from sales/revenue are distributed, and it eliminates the potential for someone trying to "switch the percentages" down the road so to speak, because it is a formal legal structure. 

I have only ever had to submit a formal plan a couple of times.

The Obama era "cole memo" which allowed state decriminalization to stay off the federal radar screen has been formally rescinded--and the new MA federal attorney has stated he has the right to prosecute all involved.

"A lease option (more formally Lease With the Option to Purchase) is a type of contract used in both residential and commercial real estate.

My identification date is Nov 9th, but nothing has been formally identified yet.  

You have gotten some good advice on this thread, but make sure to have your lawyer review and formally write up whatever agreement you and the seller come to.