From my experience with my '06 SC, and from talking to fellow '05-'08 owners, I would recommend you look for the following on an '05:
Before You Buy (especially if the 3-year/36k mi warranty has run) --
1. Check the headrests for warping (a problem with the '05s only)
2. Closely inspect the fit on all the Saleen-installed body panels.
3. Listen for brake squeal when you come to a stop at a stop sign. Saleen has replaced the front rotors on cars with brake squeal at no charge. But if your car didn't get new rotors and has bad brake squeal, I'm not sure if you'll get yours replaced once you're out of the warranty period.
4. Exhaust rattle (an '05 problem for which Saleen issued a technical bulletin)
5. Front
suspension "clunk" (Saleen also has a TB)
6. Chrome peeling on rims
After You Buy --
1. If you buy a supercharged Saleen -- go get a custom tune or at least contact teamjdm.com for their canned tune. The '05 and '06 factory tune on the supercharged cars run a dangerously lean air-fuel ratio.
2. Contact teamjdm.com (they now handle Saleen's tunes) and upgrade your hp.
3. Take pix and post them!
They're awesome cars. Good luck.