Start at 2K but go as high as 3K. If he wants more maybe more invasive investigation can help settle the question. Things in doubt are the condition of gas tank, not the paint but rust inside from sitting with gas in it. Condition of front brake master cylinder and calipers, brake fluid attracts water corrodes inside parts. Was engine fogged when it was first stored. Things of this sort can be bargaining points.
Look in the gas tank. Any amount of gas stored in this tank for 10 years is a bad sign.