I would consider converting these people to a rent credit program. The legislation for Seller financing personal property is not limited to DF. There is FInCEN and other lending requirements which in my opinion are terribly prohibitive to doing a simple contract for deed.
There is a lot of work to get these notes into compliance either way, and at least 50% seems logical, maybe more depending on their terms and if they seem predatory.
The real expert on this is Ken Rishel @RishelConsultingGroup