looking for a used, cheap shock to put on a single speed xc bike i'm throwing together.

i'm an athletic 210lbs. not going to be doing any big drops....mainly just semi-technical single track. don't want something that goes all squishy every time i stand up.

found an '04 rock shox duke sl air locally, and am wondering if it'll suit my needs?

if not, what would you guys suggest?
under $150. 80-100mm, not too heavy.