If I was in the market for a 604, I'd want muzzle velocity data just to verify function and seal condition.
A service and reseal costs around £90.
I'd also want to check for signs of fatigue on the top strap as they can snap at this point.

If all is well and MV is healthy at 370-390 fps then I would consider £250 a fair price for a Mk2 with
adjustable palm shelf.