Is it possible to get this working with Hoppie plus another version of software? I saw a post on a site dated as it was, that stated using the cpdlc client from https://www.dalpi.de/ to work with the aircraft but haven't tested it yet? Should I just drop the money on the new FSLabs A321?

There are a number of standalone Hoppie clients (such as the one you linked) which can be used, at a basic level, to send and receive CPDLC messages, with any aircraft.

However, they are completely separate pieces of software and do not interact with the aircraft itself in any way, so for example you will have to re-enter all your flight details in to the standalone client and it obviously will not interact with anything in the aircraft (so you cannot uplink ATC reroutes direct in to the FMGS, transfer radio frequencies in to the RMP etc, position reports have to be fully manually filled rather than taking information from the FMGS etc). 

Because the FSL A321 has it integrated as part of the aircraft all of this is much smoother and just forms part of the normal course of operating the aircraft.