I like Happich windows, properly fitted, with unstretched rubber seals, and the correct tension on the hinges, and fasteners, they should be water tight, and give no problems. At UD this weekend, I saw at least 5 cars with them fitted, 2 of those cars had them poorly fitted, and the rubber seals looked tired, and seen better days, there are plenty of other cars out there that use the same set up as the Happich kits, so finding a new rubber seal, that can be adapted to fit, really shouldn't be a problem, just requires a little detective work...
Thom