Thinking about starting a full army conversion project. The issue becomes if I use another faction's models. I’m looking to make Armageddon Ork Hunters which is a guard regiment. But they fight like orks. So if I convert a guard army to fit the ork codex completely. Which codex would I use the ork? Or would I still have to use guard, because looking at the army it would be like the guard looted the orks.
If you're playing casually, you do whatever you feel like so long as your opponent is happy with it.
If you're playing competitively and want to use the Ork rules, you need to stick to the normal rules for any conversions:
- No potential to confuse your opponent - e.g. you can't just stick a Taurox down on the battlefield and claim it's a Trukk, it has to be clearly distinguishable from a Taurox.
- What You See Is What You Get (WYSIWG) - e.g. if you want a model to be a Big Mek with Kustom Force Field, your model has to have equipment on it which generates a force field.
- Games Workshop models only - they can be any Games Workshop model, whether from Warhammer 40K, Age of Sigmar or something else, but your conversion has to be Games Workshop models (with a small amount of green stuff allowed).
Is this possible? Yes, but it's a large amount of work; for actually the reverse of what you're proposing, see Matthew Beavis's amazing "Grot Guard" army. It is perhaps worth noting that Matthew has annotated the bases of the characters with their official names (Commissar Yarrick etc) in order to meet the "no confusion" rules.
Of course, everything in the end is down to the discretion of the tournament organiser (TO) - if you're intending to bring an army like this to a competitive tournament, you would be very strongly advised to start the discussion with the TO long before you arrive; don't be "that guy" that turns up with a marginally acceptable army and makes life difficult for the TO.