You want the IQ Panel 2+, which as you said comes in 3 flavors. Each one supports a different "legacy" frequency mode for sensors. I was upgrading from a 2GIG system, so I chose the 345MHz one, so I didn't have to replace any of my wireless sensors. The 345MHz version is also compatible with Honeywell. The other flavors are for DSC (433MHz) and Interlogix (319MHz).
All 3 versions, however, support the PowerG encrypted sensors. If you are starting from scratch, you could really pick any of the 3 versions.
Hope this helps.