I think if you pay $17,000 for it you are paying about a $5,000 premium for the emotional and sentimental value of the seller.
Zero rust (and I mean zero, zero) would bump the price from $5,000 to $8,000. The low miles bumps it up another $2,000 to $3,000.
I'd want to see the maintenence records, oil changes in particular. There should be a minimum of 10 oil changes on a 10 year old vehicle (once a year no matter what). Any less than that would deduct $500 each.
If its in near-pristine condition it's worth maybe $13,000.
The engine (and everything attached to it) is worth $8,000, the transmission about $1,000. That's the bulk of the value right there. The same van without the engine or transmission is worth about $3,000 or $4,000.
I'd shoot an offer of $10,000 knowing (and stating) that there are a lot of sub-200k mileage Sprinters out there for well under $9,000, and see what he says. If you can get it for under $13,000 it's a fair price.