I have a question for Chas, if he's still on the board. Hopefully the question isn't too painful. What engine oil were you using when the 996 engine let-go on track?
I was using 0w-40 Mobil 1 (the factory recommended fill). some guys use 5w or 10w 40, didn't hear of anyone using anything thicker (50 weight etc) but you could go with a more racing-oriented oil (Motul, Royal Purple, etc) in 10w-40.
The oil I was using was "approved" by a professional Porsche racing mechanic who knew these cars pretty well, so don't get too wrapped around the axle about what you're using, just change it regularly (I changed it after every weekend, so probably every 300-400 miles) and inspect the filter when you do so.
btw, my engine didn't suffer an IMS failure, at least not directly. I had previously replaced the IMS as a maintenance item. what failed was the rod bearing on the #1 cylinder. it's possible that could have been a second order failure from an IMS problem but not necessarily.
also at the time of failure the car/engine had nearly 20k track miles and 70k total miles on it, and had been used pretty hard with R-comps/slicks. one theory is that the engine failed due to repeated oil starvation in high g corners.
if you go with anything with an M96 engine...3.4 or 3.6, 996/997/Boxster/Cayman, do the following:
-motorsport AOS upgrade
-X51 oil pan upgrade (not sure if these fit Boxster/Cayman)
-Accusump or some other assist for oil pumping under heavy load. I did not do this and probably should have. might have saved the motor.
doing the above is ~$8-10k with labor (motor has to be pulled). new motor is $15-20k depending on core reimbursement etc. so it's an expensive proposition either way. don't be fooled by sub $20k 911s or nearly $10k Boxsters, they'll still cost you a fortune.