I'd pay $4000 for it, if it doesn't have the original transmission still in it. If it does, $3000 max. I'd also pull the cover off the top of the engine and look for any signs of Black Death. If there is any of that, $2000 max.
Dealer retail on an 05/06 Sprinter with 450,000 miles is between $10,000 and $12,000 if it's in good mechanical condition and the body doesn't have very much rust on it. Wholesale is about $8,000. But, again, if it still has the original transmission in it, that will be the first thing you'll need to replace, and that will be about $3000, maybe a little less, for a Silverstar ($2300 including shipping), including the installation ($500-$700). If it still has the original transmission, though, that's a high indication that it's been well maintained.
I'd also want to see the maintenance and repair history, especially oil change history, including type of oil used.