Try for a personal loan, most banks and credit unions loan up to $15k. With the $8k you already have, you should be able to find a truck in the low $20s. Owner financing on a vehicle is a crazy idea for both parties; Seller has to be sure that the truck is kept fully insured (incase the buyer trashes it and decides to walk away)and the buyer will probably end up with a nutty interest rate to make it worthwhile to the seller.
Capital One (depending on credit) will finance older vehicles. Another route, lease with option to buy, Copley
uses a company called Putnam Leasing.
Putnam Leasing from Greenwich, Connecticut. Putnam Leasing has been
leasing vintage, collector and exotic cars for nearly 20 years and truly
understands you and the special car you want. Putnam offers long term
leasing, low buyouts if needed, no penalty early termination and collateral
Keep in-mind that affording a Defender just doesn't end at a monthly payment!