I guess what everyone tends to recommend is to buy a used one that is 'broken in' and has already had it's problems. I think this one's up to you. I've basically rebuild mine, which is a 1986 Beck that was build and never finished, ie; 300 miles in 17 years.
You will probably spend as much building as buying but there's a lot of pleasure in building one, I would imagine since I really haven't build mine. There are so many upgrades and touches you can make that you'll likely never stop. I would suggest going to the Spyderclub.com site and looking at www.spyderclub.com/phpbb2/viewtopic.php?t=2638&highlight=top+things and www.spyderclub.com/phpbb2/viewtopic.php?t=2074&highlight=top+things and going from there for things people like and want. You'll get great parts and service from Dario.
A few things I like are 4 wheel disk, I'd like to change to wide 5's (because I like the look) but am waiting for some $, small plexiglass windshield (but do this after registering the car, just in case), and so on.
I would do everything you'd like to first and then paint it.
Also I've done quite a bit of Solo II'ing with it and it's a total blast. For Autocrossing I would recommend a good second set of rubber (205/50's minimum), coilovers if possible (sorry, not for me with a Beck) and sway bars. What I've learned though is it's a lot of fun hanging the rear end out and catching it (sometimes!), so you really don't need to go to the extreme's.
Also, as always, you can never have too much horsepower. I'm one of the very low HP guys that I hope to change soon also, but I just need to keep my foot into it longer!
RSK or 550? Go with the look you like.