...type of diving and personal preferences. The undie market is such a nice little niche, that they seem to unapologetically taking advantage. So, I feel your pain.
You've got the first thing right, in that regardless of what you use, it needs to be a synthetic/wicking fabric. Non-diving specific layering pieces, fm ski shops for example can work, but the question is, will it work for you and is it the best for the job?
When deciding what to wear, of course you gotta consider
1) what type of diving you do
2) how cold you get
3) any weird personal preferences...to which I have plenty.
My answers
1) Avg temp's 40-60, current max BT's 60-70 min, foresee it progressively increasing over the next couple of years
2) Average
3) I hate two pieces (bunch's up and can leave a cold spot), I like stretch, and as a member of the fairer sex, it's got to fit well.
What I've been wearing: DUI's actionwear.
What I'll be wearing in 2 weeks: custom made 300S polartec jumpsuit for appx. $100
Future purchase: 400G Thinsulate or equivalent.
(All of which I'd layer w/ polypro's.)
The areas where the non-made-for-diving undies lack are:
availability of one piece
availability of thinsulate
no attention paid to the amount of added buoyancy
high necks are often used in the tops, and would need to be altered to not interfere w/ the neck seal
So, are these things worth the prices they charge? Probably not, but the good news is, is that there's a lot out there for comparison.
Christine