thutch,
Not sure if you are still looking for advice, since it's almost a month since your post.
I would suggest to you that the first place for you to start would be to have your renters pull their credit report so you know what their credit is like. Make sure they don't pull an infile report. It needs to be a tri-merge. This way you will have their score from all 3 reporting bureaus. Once you have done that......you will use the middle score as their "actual" score. In other words.... if the credit report shows their scores as 600, 540, 580.....the score you would use would be 580.
It is possible to qualify for 100% with a score of 580. It might be an 80/20 combo ....or it might be "one loan" Either way....it can be done. Their debt ratio is another factor, but you can have a ratio as high as 50% with no down payment. Or a ratio higher with a down payment.
Like jespah said.....try to get them to finance on their own if you can. If they don't have a down payment, you can always do a BACK dated land contract. Then it becomes a refinance instead of a purchase....and no down payment is needed. And the good thing about doing that is they will have a mortgage history showing no lates.
If all else fails........you don't have to have your mortgage paid off to do a land contract. If you go that route it's fairly standard to make it for 24 months....with the hopes that they have their credit cleaned up by the end of the contract.
If you have any questions.....you can pm me
~Brooke