Most of the pricing at this point is on condition. $21-$23k seems fair depending on how well he took care of it. If he's got loads of receipts showing care, and it shows, then I would be more likely to pay a little more.
Look at the options though:
Tower -- Is it a New Dimensions tower? That's the nice one that looks good with this boat but really expensive to add. Like $2k-$3k for tower itself.
Perfect Pass - Does it have it? That would be a $800 to $1000 addition doing it yourself. $1400 if a dealer installs.
Trailer tires/brakes/bearing in good condition? If you have to refurbish then that also needs to go into the equation.
Boat prop - Original or aftermarket (such as ACME or OJ)? That would be another $400-$500.
1994 205, LT1