If you know the guy and are friends, and would like to remain friends, I suggest you get EVERYTHING in writing. Agree on the sale amount, and the down payment, and then write out a contract
"I, ______________, am selling my ____ Jeep, VIN__________________ to ___________ for the purchase price of $12,000 and a welder. This includes a down payment of 6,000 and the welder. The remaining 6,000 will be paid to me by Bob over the course of the next 12 months at the rate of 500 a month."
Include all the other stuff, you know "selling as is, seller not respnosible for smog, etc. etc. etc." whatever you agree on. After you're done writing and you both agree, both of you should sign two copies, one for each of you and date it. This way, if a problem ever arises and for some reason this gets dragged into court, you can show that both parties were in agreement.
there really isn't a cut and dried formula for making a contract, you just need to include a timeline, and the down payment, and the amount of the payments if you are willing to sell it that way. Make sure you include the welder in the contract as part of the payment.