Man, that is a clean looking truck.
If it drives well, no blowby, no jerky steering and its clean underhood, with batteries & cables looking good. take it for a 20-30 minute test drive. watch for smoke and odd noises.
Try to find an independent diesel pickup repair shop and pay for an hour and have them inspect if you are unsure of what to look for.
Then go back and offer $20.500.
West coast prices seem to be higher than many other places.
MyCarFax lists my 2003 slt 2500 with 133k miles for private sales at $ 19.250.