a low mileage '05 with decent options and get the IMS done immediately. So you have to pay $1.5-3k for the LN IMS, you are saving $15k on the '05 versus the '08 and have the same basic styling and the no IMS-inner-seal design by taking this approach. The '05 is usually the easiest/cheapest IMS replacement there is on a 987. (I say usually because there are rare exceptions.)
Recall that in 2012 there will be a new model out...probably available mid 2011 so any 987 will be one model dated almost immediately. That means a drop in value for the 987s so you have to be careful not to overpay when buying any current model.
If you are looking for a CPO, look very carefully at the transferability of the CPO. "Once the covered vehicle is traded or sold to any non-authorized dealer or commercial broker, this warranty is void."
Good luck....