Need Help With Car Contract?

Im buying a car from a private party which will equal up to $7800. We already have an agreedment which is I will drop $2000 up front and then pay $1000 a month until I am done. I will be giving them 6 personal checks with written amounts & my signature but without the dates so that all they have to do is cash the checks once I give them the go ahead every month. I will not get the car until I have finished paying it off in full. The agreedment also includes that they not put any extra miles on the car after I make the first payment. Can someone please help me in making this contract or is there a website that helps out or does this kind of thing. Someone also told me to get the contract notarized at a bank but Im not sure if you get it notarized before or after the signatures of both parties. Also should we both make copies of each others IDs. I have meet the couple selling the car and they both look trust worthy. Please any details anything that can help me would be appreciated. Thanks.