what size horse power is the motor? You can put a $1000 into a motor and still have junk. I have seen it. What did his $1000 fix on the motor? Try to get some detailed photos of the boat, motor and trailer and post them. Do you have to re wire the boat is everything working? Is it clean and orderly? How does the owner appear to you? Clean and orderly?
If the boat is sea worthy (meaning all you have to do is buff it wax it, tune the motor and when you connect a good battery everything works) and you have reliable HP @ 75 and above (make sure he starts it for you) wave $800 to $1000 cash in front of him whatever you are comfortable with. If he says no then from there what is having this boat worth to you? Be sure he proves everything on the boat for you. Every dollar you give him is that much less you will have to get it useable and running again.
Motor
Water pump impeller, spark plugs, gear oil, carburetor kits, thermostat (if OMC) plus either your time or having to pay for someones time to tune it up. $300 and above
Trailer
do the lights work? How are the bearings (last time replaced)? How is the hardware on the trailer (fasteners for axle etc...) are the tires rotten? could also be $300 and above
A boat sitting out in a yard like this for 2 years will take quite a few steps backward from when it was last used. All the glass needs to be re-polished it is all hazed over. Trust me I love the Dorsett and they are getting rare but more details need to be provided to make a better judgement and it is up to the owner to show you the true condition of the boat. Ask for receipts and so on. Walk through it bounce check the floor and transom real good and be sure everything works.