Hi,
That is what I used pretty much on my first attempt: a lycra (spandex) suit and I sculpted muscle and armor sections out of upholstery foam and adhered it to the suit. Then, I applied about 4 coats of latex on top. It didn't work out too well for a few reasons.
Here's the problem:
The latex will basically be absorbed into the wet suit you will be using most likely, causing whatever you have to fill the suit for form to stick to it. This makes it extremely difficult to remove the suit from your base, and it could lead to massive paint flaking. However, the main reason using a lycra suit didn't work out for me is that it shrank WAY too much, even though I used a duct tape dummy.
Could using a wet suit work out? Possibly, but you would need to stuff it with a TON of either newspapers or packing material to avoid significant shrinkage. When you start working off a spandex or wet suit, it's all an uphill battle for the most part, because you have to sculpt all of the muscles/armor as well as contend with the shrinkage/flaking issues I mentioned.
Honestly, my suggestion would be to take a superhero muscle suit and add your foam bits and paint your latex on top of that. This is for two reasons: the suit will not be as porous as the spandex or wet suit in your case and will keep most of the latex on the surface. Secondly, with a muscle suit, you will already have muscle groups to work with instead of having to sculpt everything yourself. Also, you can get away with using MUCH less latex- I only had to do 3 layers of latex for each side of the superhero muscle suit and it created an excellent surface.
Finally, superhero muscle suits in most cases are cheaper than having to buy a wet suit of some sort I would imagine.
Just my two cents, you can go to the Non-Predator Costume section and see what I'm doing with my mark II suit I'm currently working on.