just buy the damn car. You can get an awesome 986 for 17k. I'd probably look for one in the 12-15k range and buy new tires / clutch / brakes / IMSB / suspension upgrade with the difference though...
Can you work on cars? Do you have lift access? If so, it's very easy to get in there and swap out the IMS bearing. After having pulled my trans a couple times, I think I could do the whole job (clutch and IMSB, drink a beer) in about 4 hours start to finish.
Otherwise, don't sweat it! I sold my car for a little less than what I paid + parts I put into it (I don't count my labor, otherwise I'd be way behind) ) and a had a blast with it! Chicks / dudes / kids / dogs all love it, and most importantly, YOU will love it!
Just make sure it hits 5k RPM at least once every time you drive it and the IMSB will be fine
Best car decision I ever made -- the only reason I upgraded (to a newer 3.4 Boxster S) was because the price was right.