I have owned a 2007 21V for about a year now and it has been a great boat I have the indmar 325 in mine, which seems a little under-powered at times. What engine is in the model you are looking at? How many hours? If you are expecting skiing at 36 that will be at the very top range of the boat's capabilities, unless you change the prop. The only engine problem I have had is vapor lock. Based on what I have heard every single 2007-2008 Supra has this issue because they mounted the fuel pump directly to the engine. In 2009, they put the fuel pump in the gas tank so its not an issue. I solve my vapor lock issue by draping a wet towel over the fuel pump when anchored for swimming, works 100% of the time. The only other complaint I have is the gas tank size is only 30 gallons, which is small for this type of boat.
As far as surfing goes, stock it is not a great surf boat. I have 400 pound bags on each side in the rear and an 800 pound bag in the center. I can not get the wake right, the wave is not steep enough at 11-12 mph to support surfing. I have heard guys loading extra sacks in their boat get it right but that is not very convenient when you have the family out
. It sounds like the boat you are looking at is setup well for surfing with the aftermarket gates...etc. I would ask about the ballast pumps, see if they have been replaced, the stock system is SUPPPERRRR slow to fill and empty... its a 20 minute process filling them.
If you have any other questions feel free to ask.