if you do get a used rex I'm sure some of the WRX have some advice on what to look out for, but make sure you get one in good shape. Also as for options I'd say get a boost gauge.

 

I used to have a 2.5RS, and now I have a WRX, so I can offer some comparisons for you:

In favor of the RS:

1. You could (probably) get a well-equipped RS for 20k or less. I got mine for a lot less than 20k two years ago, but I had to use the Sam's Club auto-purchase program to get it that cheap.

2. In my opinion, the RS is a little better suited for auto-cross type driving (engine wise), since you don't have to wait for the turbo to spool up to speed. However, and this is a big 'however', to do auto-cross w/ the RS the rear sway bar would be the first thing to replace.

In favor of the WRX:

1. There are a huge amount of aftermarket parts available (as others have already pointed out) for the WRX. That was what really pissed me off when I had my RS; the WRX people had all these go-fast-goodies to choose from, and I had to make do with whatever happened to fit both models. Grrrrrrrrr.

2. World Rally Blue is a much better color the Blue Ridge Pearl.

3. You get a free 1-year membership in the SCCA with a WRX.

To summarize, if your main consideration is the initial purchase price or quickness on an auto-x track I'd go with the RS. If you're more interested in the ability to do modifications &/or top-end speed and power, save up for a while and get the WRX.