A wetsuit for caving is always going to be a tradeoff between mobility and warmth to some extent. I like a 3mm one piece, and recently bought a new one, although I had to get a custom made suit to fit me. I got mine from http://www.aquaflite.com
, they were great to work with, very reasonable on cost, and made a nice suit. They even altered it and sent it back at no cost when the height was off a bit, probably the measurements I sent them.
You may want to consider some options like getting a 3mm or 4mm suit and a separate vest that you can pull over the top of it for extra warmth in your core when you need it. There are also suits where different parts of the suit are different thicknesses, like a 4mm body and 3mm legs and arms for a combination of warmth with mobility. It's really going to depend on how hot or cold you tend to be individually as well as how much you're in or out of the water. For instance, I *have* to have a suit with a front zipper so I can vent when I'm out of the water.
Personally I would spend more to get a suit that fits well, the extra comfort after being in it for 12 hours or more is worth it I think. If you can fit stock sizes well, consider yourself lucky as you'll come off a lot cheaper being able to take advantage of used deals and such.