The 21' Daytona with V-8 power was a popular set up in the mid to late 80's and early 90's. They are good boats and a lot of fun. You have had some good advice though. I have been told by several people that there are a few main problems with them....they suck fuel like no other, their gearcases are week and blow easily, they won't turn many rpm's, and parts are harder to come by.
I think if you leave it stock, have the bearing carrier pinned, make use of the torque and don't try to spin it more RPM's than it is capable of, you will be alright.
Having said all that, it is my opinion that those boats are worth about $12K in perfect shape. Face it, the V-8's don't have a great reputation and you WILL end up putting money into it. That makes them less desireable and harder to sell.
If that boat has some driveline issues, I would offer him $8K - $9K and come up to $10K max.
Good Luck.