What makes a good ski boat?
Small wakes
Soft wakes
Less Spray
Better Tracking
Smooth and consistent speeds / times
How much you care about each of these depends on you and also on your level of skiing and on the level of the driver you have access to.
A wakeboard boat will be very bad at pretty much all of these items. Don't even consider it.
The oldest/cheapest boat I would personally even entertain would be the 91-94 MC prostar 190. No wood. Great wakes. Small soft wakes. At your level, you don't care that it had pretty big spray. Also at your level, especially if you have access to a pretty good driver, you don't care that it doesn't have as good of tracking as newer boats. You slap perfect pass on it and you are a really happy camper.
The next oldest boat to consider would be the TSC1 hull nautiques starting in 97. No wood, small soft wakes, even the spray isn't bad. Pretty good tracking and PP speed control is again, good enough at your level of skiing.
The next oldest would be a malibu response with the diamond hull and no wood, somebody will have to help me know what year that starts on.
Boats created after those 3, don't really do any better at the 2 things you probably care most about at your level, which is a small and soft wake. The next 20 years were spent largely in a pursuit of reducing spray, better tracking, and bigger boats overall, and then the switch to ZO speed controls. All of these are things that matter a lot to some people, but I would doubt that they will matter all that much to you right now. If you're getting into 38 or even 35 off and you are skiing tournaments, then ZO really matters, less spray really matters, better tracking probably even matters even if you do have a really good driver just because you are cranking on the thing so darn hard. Shoot, apparently if you're into 38 off, then you think the new microtuners are awesome as well.
For your purposes, I think you're mostly interested in a small soft wake at your slower speeds an longer line lengths. For that, the 20 year old boats mentioned above can pretty much do just as well as the new boats.