I got lucky and found one that had already had the IMS replaced.
I was on my way home from seeing another POS Boxster and determined I was giving up my search - when the guy called me (returning a call from a few weeks before)...keep in mind, my search took 18 months (I fully concur with what njbray says above about not rushing!!)
The guy was an attorney general and spent all kinds of crazy money on my 2000 S - the first owner had it 11 years and only drove it 30k...this guy spent over $6k in 2 yrs/20k miles. He repaired all the normal stuff, plus a clutch, RMS and LNE IMS (and he did the 60k checkup at 50k, since the car sat a lot - yeah for me!!!) He had records for everything he had done and carfax corroborated what he told me about the first owner.
I made an offer, before seeing the car in person, and drove 300 miles to get it. No PPI. I knew I was gambling, but this guy seemed trustworthy as well - heck he was an AG - I could not imagine he would risk his reputation by trying to scam me. The real gold came when I was able to talk to the shop that had done all the maintenance for him - Import Performance in Wilmington...they gave me solid info and a full print out of all the data they had on the car...jackpot!
I am a little suspect of PPIs if the car is already in pretty good shape. I am well versed in mechanics, having owned and repaired over 30 different cars in my life - including having rebuilt several 914 type IV engines- so I took the chance without a proper PPI. I think Jake Raby is the only one who can do a PPI I would pay for, but he does not do them any more
Unless the PPI includes a Blackstone report, oil filter inspection, leak down test and paint test, I just would not have a lot of faith in it.