[JuanPC]

Well,

10.10 & PT12.4 not a good idea...

You can always install bootcamp & Windows7x64.

But PT12.4 no go.
12.3.1 works well, but needs a very powerful machine.

Usually...
PT12 needs 2x drives, as fast as possible.

PTHD12.3.1 can live with one SSD, at 6Gbps, "500MB/s."
And a very large Ram Cache, 20Gb ram ECC.

Don't know if iMac 5k has ECC.
PTHD Ram cache will work without ECC, but if memory fails 1bit, could loose all data.
If you work with video and audio, could loose sync...