So it depends on how rules vs fluff you want to go?
To me, Mk IV is the most advanced form of Astartes armour in the Heresy and so is used by units that are more all-rounders. Mk VI is newer but has specialised systems meant for infiltration etc and interestingly MkIII was re-adopted by many legionnaires as the front plating is more resistant to Bolter Fire, an unexpected side use when fighting other Astartes.
Based on this, I say you go for a variation on Imren's idea - a Flamer Support Squad (as opposed to heavy flamer) and also give them a Bolt Pistol + CCW (which you can do!)
This way you're getting extra attacks, can sprinkle in the chainsaw look you desire and use the MkIV in a meaningful way!