Is this a P or an M version?
Assuming it is a portable, I note from the latest software release bulletin that you need at least 256 MB of flash in the radio.
To find out the flash size you need to use the RPM Radio Maintenance utility, and select the Radio Information button. The flash size will be displayed under Hardware Info.
The P has no restrictions on band, type, part number or hardware revision other than the flash size. The M lists various part numbers and revision levels by band that can support this.
I thought I saw some info on the voice file types but I can't find it now.
To make the radio receive only, you go into the P25 Trunked system setup screen and on the right side there is a section called Auto Registration; use the drop-down box to select "Never", save and reprogram the radio. That should do it.