I received my pre qualification letter with these conditions below.

The benefits to seller is that they can collect additional income via interest on the loan, potentially a higher purchase price for the use of seller financing, and they can spread out their capital gains tax exposure (if any) over two or more years rather than all in one year.Benefits to buyer using seller financing - ease of loan docs and no lender qualifications, no appraisal required, speed of financing, additional leverage avenue, potential to have lower interest costs than a hard or private money loan, no points or loan fees, etc.The key to seller financing is how you negotiate and how comfortable you can make the seller with the scenario.

Pre Qualification process and the deals you've done?

Assuming a loan requires (1) that the original mortgage allows for it which is rare these days, (2) the buyer will have to meet the same qualifications as getting a new mortgage, (3) there are closing costs and (4) often the interest rate is much higher than current rates which means the property will cost you more. 

No lender can require that a borrower remain in the property forever over the term of a loan except for government liens that might be financed under low income qualifications (for example, a city finances a sewer project to a low income homeowner with a forgiveness of debt arrangement, the debt may be forgiven so long as the property is occupied). 

And would being married affect our qualification for a second FHA loan since his current mortgage (with only him on it, but once married would be jointly owned, correct?)

does anyone have any special way to qualify a cash tenant in a low income area? I have my techniques but would curious to know everyone else's. Please share

Find a lender first, get a pre-qualification letter, and submit that with your offer.

Yes, it is possible, to a degree but to do so you need to take into other considerations as to the local market, property condition, credit issues, qualifications of the borrower and ability to reduce the UPB, the term involved and issues of predatory dealing which is an issue in commercial lending as well as residential.Big misconception of money types and dealers is that if my deal is a "commercial" transaction, I'm free to do whatever as commercial is viewed as being between sophisticated parties.

And you won't have a balloon in 2 years either, even if you could that might not be enough time to cure your qualification problems.If these matters apply, sounds like they do, consider a lease for 3 years or more.I say, in this case more because it's non-owner occupied and the seller has no exemption on the sale (but he will have more depreciation to reclaim) and there is no mortgage with due on sale issues.