Depending on how many hp's your interested in pushing, I would recommend aftermarket motor mounts. Many people pushing mild boost keep the stock mounts, but I doubt they will hold up when the going gets tuff. Since your already paying alot for the exhaust, you should have the shop make the downpipe. Also think about having them make a cat "straight-pipe" replacement for those days at the track. (Turbo's love to breathe) For 500$ these mods should not be a problem. Air/Fuel, Boost, and Oil pressure gauges are good for starters. The more ways you can monitor your engine's status the better. Also consider moving the "inter-warmer" to the front. It will increase performance and save you the hassle of cutting up your hood. Unorthodox Pulleys for the SR should work just fine. As for the clutch...???? Depends on how aggressive you will drive your newfound friend.
I read that Chris from SR20Development with the 200SX was on something like tranny #12.
Maybe you should save up for a couple of them too
$2000+ seems alot for the install.......shop around a bit.