The CZ527 is a good platform for this build. The donor rifle should be a 223. All that is required is a barrel change and a simple magazine mod. No feeding issues like those experienced with the Remington M7.
Even though you don't intend to shoot subsonic, I would still barrel it with a 8 twist barrel. If you ever decide to sell it, it will be much more desirable if it will stabilize 220's.
I have not shot mine with supers, so I can't vouch for accuracy with them. Subs will shoot just under an inch with the load it likes. Hodgdon has data on their web site. Best powders for supers are H110/W296, and Lil Gun. H4227 also deserves a look. Stick with bullets no heavier than 140grs. for supers. By the time you get to 150grs. velocities drop to 1900fps.
Cases can be made from surplus LC brass, or you can buy them from several sources that are now converting them.