Oh gosh a complex and tough question. My first concern is how well these homes are marketed, especially in a very competitive buyers market. Are you just marketing them on your own or did you pay to list them in the MLS and places like realtor.com and other common sites for broader exposure? Are you willing to pay a % to the buyers realtor for bringing the deal to you? It should be worth your while - you need as many people "selling" for you as possible.
Any decent real estate lawyer should be able to write up the contracts for you - this is just standard contract law after all. The best part of seller financing of course is that if they default that you get your house back - but gosh only knows in what condition. In theory, their down payment to you off-sets some of this risk.
This is cheating a bit but you may wish to ensure that they are listed as single family homes, rather than townhouses - to help ensure a broader audience for people looking. Townhomes are often just as good to many people as single families - provided the association fees are reasonable - especially if they are a low quarterly #. A LOT of agents list townhomes and even condos in this way. Should they as professionals in the field - well ... no - but it happens quite a lot.... more