Several people have had problems using the serial connection to the CM11A with XP.
AHP that uses the CM15A blows away AH that uses the CM11A. That's why there are so many deals for the CM11A with the old software.
I would agree that AHP with all the plugins is much more powerful than AH.
However, I have used the CM11A with at least 3 XP systems with no problems. I would guess that most HAL and HomeSeer users STILL use the CM11A (since the CM15A is NOT supported) on XP systems.
Frankly, except for here, I haven't heard of any CM11A problems that are specific to XP.
Anyway, the OP asked for "the cheapest possible", hence my reply.
Thanks,
George