It's a buyer's market right now. Research book values on anything you find - and don't even pay book. Even the diesels are taking a bath - they do NOT get great fuel mileage.
You also do NOT want to finance a used truck heavily - the values are declining so fast you will be upside down for a LONG time.
Book value on a 2000 crew cab short bed 4x4 F-250 XL diesel with 100k on it is around 12 grand. A more realistic price to pay for one is 10k.
2003 F-150 XL RCSB 2wd 4.2 M5OD 3.55 LS