I bought the 345mHz panel so it seems like the powerG hardwire unit you sell is my best option? If so, can I add two expansion cards to the unit to give me a total of 24 hardwired sensors?
Yes, you could use the PowerG Hardwire translator or the 2GIG TAKE-345, but I would definitely recommend PowerG.
You could use two expanders with the PowerG Hardwire translator.
If you went with the 2GIG TAKE-345 you would need 3 of them, and they do not have a battery housing, charging circuit, or power supply included, have less range and no encryption.