Looks like you are using 319.5MHz sensors. If buying an IQ Panel 5, those can be reused so long as you include the IQ Card 319.5MHz QC0001-840 in the purchase. The IQ Panel 5 also works with PowerG sensors.
Z-Wave devices that are working on your IQ Panel 2 should work on the IQ Panel 5 without issue.
You can use the Alarm.com smart thermostats but you are not limited to those. The Honeywell T6 Z-Wave thermostat can be used as well.
A quick comparison of the latest ADC thermostats and the T6 can be found here:
Qolsys has a list of tstats that should work with the system listed here too..