Good idea Aristeo; I hadn't though of different marks or armour.
However, after some consideration, I was helped by noticing that the accursed Ultrasmurf Chaplain character

was T6 due to bionics.
So, I'm planning a squad of apothecaries led by a techmarine, all augmented with bionics to count as the T5 and FNP plague marines (the bionics could also account for the I3).
Fluff wise, they'll be a battlefield salvage squad tasked with retrieving geneseed from fallen friends and enemies alike, and also with the appropriation of enemy tech or other useful stuff in general - also a perfect excuse to get them to sit on objectives whilst they carry out this work
I've got a load of hooded heads to use, but I've also got a load of Bt robed fronts I was going to use. My next question is; Would painting the BT cross red on white robes be too cheesy?