Now, there are two ways this could be done: As a suit of armour or as a vehicle.
It's obviously a vehicle in Dark Heresy; it's to large to be power armour, and the existence of hard-mounts means it's a vehicle, as it is self-powered, the users speed is irrelevant.
Except that crisis armour is not actually very large - according to IA: Taros Campaign they stand ~2.8 metres tall - that's only a 1.5 feet taller than a marine, and their suit is written as armour. Effectively, Crisis armour is to a marine what a marine is to a human. Terminator armour would be almost the same size as crisis armour.
Hellebore
You can actually model power armour as a "vehicle," since it is in essence all but a mecha battlesuit. Indeed, I'm constantly torn on whether to model it as a battlesuit/vehicle, or whether to take the "suit of armour" approach. Erm, but there we go.
Kage
I'm also wondering whether when writing one up as an enemy, it might be easier to use ordinary stats to describe it for ease of use.
Erm, pardon? By "ordinary statistics" are you talking about the traditional add on Hulking, throw in some strength bonuses, an armour rating, etc.? Or something else…?
Kage
Yep, that would definitely be one of the easiest things to do. As long as you've asked yourself whether it would be the best/most interesting thing to do before you do that, things will be good…
Kage