I've been using the Santi undersuits for the last year, and liked them so much that I've started selling them
So that's my bias, having said that I've found that a 400gr Thinsulate undersuit with an addtional layer of 200gr thinsulate on the torso, and then using argon is good for a few hours in 4 deg water while scootering. I suspect that you will need to think very cafefully about dive times though as you will need to have plans in case of suit flooding. That's one advantage of the 400gr thinsulate using argon, it holds it's insulation properties well even when completely flooded.
The other thing to think about is making sure you have new undergarments, as they lose insulation over time.
HTH
John