So its the Grey Knights stuff you're interested in running? Because my first thought is that you simply run an Inquisition army.
There are a couple of conversion armies and more converted single models from Plainville's Thursday regulars, so conversions aren't an issue. However, none of them are making this big of a conceptual leap. Usually, its power armor 'counts as' other power armor.
The most important thing is that any army is clear to your opponent, so you at the very least need consistency and coherency. Any models should be as close to "what you see is what you get" as possible. If you can do those things, you should be fine for any casual game.
If you're concerned about running in tournaments, I would make one squad as you want and seek approval with the tournament organizers. If that gets an okay, proceed forward. Once completed, check everything again with the TOs.
I am thinking Ogryns could make decent IG-themed Terminator substitutes.
