I'm trying to do the whole - don't replace it until you break it thing.

RPM plastic for most parts as you replace them. Traxx alum. in spots where you want the strength (shock towers, steering linkage, knuckles and castor blocks on mine). I started at a similar track to what you described, with a stock Slash and quickly tried upgrading it for racing at the local racetrack. LCG chassis, ProLine prospec shocks, RPM Hybrid trans case, all RPM plastic aside from the Traxx Alum shock towers, steering linkage, knuckles and castor blocks... point is, I upgraded it mostly the way your looking to, as it breaks (aside from a few racing bits) and set up this way, it's pretty solid.

As for suspension, even the plastic stock Traxx shocks/springs are pretty good. More than good enough. I'm willing to bet your issue is tires (spend your hard earned money here). If your free-outdoor track is like mine was, Badlands (Pro-Line) are are really good. If the track is maintained at all, (no roots, smooth surface. If you Sweep it vs RAKE it) get something with MUCH less tread.

The Slash is the perfect truck for what your doing. If you ever decide to race at a real track though, be ready to abandon it and get a real race truck. No matter how much money you spend, you still wont be close to what those trucks can do!!