In my opinion, Stiilus has touched on one of the main things that has to be kept in mind when constructing an undersuit: breathability. I would be very cautious with using latex for the undersuit, as it is not in any way "breathable," and that means it will hold in all the moisture (namely sweat) and with it, the body odor that accompanies said moisture. Also, latex isn't absorbent, which means that swampiness is going to stay locked against your skin, and that's going to get nasty pretty quick (and you may well unleash a stank that can stun an adult bull moose when you peel that suit off at the end of the day).
Relying too heavily on foam won't be much better. Granted it is more absorbent, but it's not a good "wicking" material, so instead of having all that moisture sealed against your skin, it's basically going to become imbedded in your suit.
Spandex/lycra and similar fabrics that are characterized by their elasticity are probably your best bet. They come in many colors and "finishes" that can make the material look rubbery, leathery, metallic, glossy, or dull. You can incorporate thin EVA foam (craft foam, or "foamies) or you could even use thin foam padding (similar to upholstery foam), which can be essentially "carved" into the desired shape, and then sewn between layers of lycra/spandex to give the suit shape while still being more breathable than EVA. My thought has been to use a "heat gear" type underarmor (the type made to be worn in warmer weather) and have as many of the undersuit details be attached to the hard armor (for example the undersuit "padding" along the waistsband of the Reach cod/belt assembly).
Whatever you go with, though, be mindful of where you can get away with, as Stiilus suggested, patches of open, breathable material. If you have a full latex/vinyl undersuit, and then put EVA/fiberglass hard armor on top of that, you're looking at a high risk of basically smothering yourself in your own suit. For Halo armor, the chest, back, shins and thighs are good places to put in as much mesh as you can get away with. A patch of breathable (but still modest) material behind the codpiece is also advisable, as that is one of the areas of the body that generates a lot of heat and will feel very, very uncomfortable to say the least if it gets that "swampy" feeling.