I'd say your odds of finding an S for under 12K is doable. I see them on craiglist.com from time to time and I was just browsing RennList today. Here's a 2000 for 11K. It sold already, but the point is they do come up ...
FS: 2000 Boxster S - Rennlist Discussion Forums
I bought my 99 for 12K, but did so knowing how clean the car was with lower miilage (67K when I bought it) with a carfax showing a history of Porsche maintenance and repair (potentially including the RMS - maybe IMS too). In any case, I really like the ride and am happy with it. Had I of known what I do know though, I'd have kept looking for an S until I found one I wanted for under 12~15K.
I forgot to mention that when searching, get the 30 day cardfax offer. While carfax is not perfect they do offer a buy back guarentee, plus if the car was in an accident the carfax might show it. Many of the cars I was considering I dropped as soon as I found out it was in an accident. Generally speaking, these were all lower priced models at a local small lot dealer. I wanted a car, but not a problem if I could avoid it.