As with any performance car look for any aftermarket parts, under the hood for things that might've been moded and return to stock, check the heatshields, look at the downpipe, aftermarket exhaust, and just normal things that look like a younger driver owned it.
Everything should be in working order with only 40K on it, have a mechanic look at it.
Also look in the forums area for the link to the used wrx/sti buying threads.
I'm asumming that it is a wrx limited (due to the leather and heated seats) so it should retail from 15-20k I bought my 06 wrx for 15995 about 5 months ago and I love it to death!!!
Anywho welcome again and enjoy your new car as well as the club!!!