You could talk to Brian at RarelyL8 about modifying one of these mufflers to work with the stock 993 3.6 heat exchangers.
http://www.mkexhaust.com/products/custom_rally.html
When you want to be quiet, plug the center outlets, and unplug the side outlet. When you want to be loud, unplug the centers and plug the side outlet. It's very analog, but very versatile. The 993 heat exchangers are the last dual outlet exhaust Porsche designed for an aircooled car, so they are pretty good. I"m sure you could make this work for way less than your budget.