Now my Ork Kill Team is mostly complete, I'm contemplating starting a second faction.

I'd like to give Heretic Astartes a try. Word Bearers to be more specific (really like their story).
However, since all Hertic Astartes started out as regular Astartes back in the day, I was wondering if I could do two Kill Teams at once: the same faction pre and post Heresy.

I've been looking at the right power armoured models but it seems GW no longer sells MKII troops.
They do sell MKIII. Would this be acceptable from a background perspective?
And what about other troops: scouts, terminators, ... do proper models exist for that?
I guess some troop types available now weren't around during or before the Horus Heresy.

So looking at the list, these are my options:
  • No Reivers, Vanguard, Eliminators or Intercessors (rules could be used for "counts as" Gal Vorbak before the Dropsite Massacre?)
  • Scouts => Recon Marines
  • Sternguard ok
  • Veteran ok
  • Tactical Marines ok

Would it be feasible to run a Kill Team like this? Would it be fun to play?
I'm still fairly new to the 40K setting in general so any advice is more than welcome