ISO Series 2 non-R10 comparision/review

[slinky]

I'm going to be searching ebay for a Series 2 DirecTivo that I can use to complement my existing Series 2 unit in order to use HMO, TivoToGo, etc.
I've noticed a couple of units there.
My question is.....does a resource exist that will tell me which units are Series 2 and whether or not any particular units have any issues that might make them a bad choice?
Thank you much!
Slink