If you just want a tower, buy a tower and install it on the Crownline.
If you want to replace the boat with a DD or V drive, just remember that a DD is not usually optimized for wake boarding - they are for skiing. Now you can "fix" that with ballast or things like the Wedge that Malibu puts on most of its boats. A DD is going to be much harder to use for wake surfing than a V-drive because of where the weight is in a DD and how much you will have to add to counteract it.
30-35K will get you into some older V-drives and plenty of direct drives. I'd take a look at things like Moomba, Malibu v-ride series, maybe Axis - but they have not been around as long so you may not find one old enough to be in your price range. Those will get you the most bang for the buck, they just won't be as fancy as a Mastercraft, newer Supra or non v-ride Malibu.