There is no M.2 card that houses a GPU directly. You can get a M.2 -> PCIE adapter, but that defeats the purpose. Just use Quicksync instead

If you're not going to use the features of Scale that Core not has, i'd suggest you'd pick Core. Perfomance wise it's better in almost all the basic Truenas tasks, untill you try to utilize the more advanced features/plugins.

If you wanna test ZFS replication, just get two nucs or other micro formfactor PC's. Unless you don't pay the electricity bills where you're going to power the. Those are slow and power hungry