You can wear it without underlayers at all. That's what I do, though once in a great while I add a 1.5mm neoprene vest underneath for added torso warmth. Anything that goes inside had better fit tightly, because it's going to get clammy from sweat which would be nasty all bunched up if not a tight fit. Also, you want to avoid gaps that let water come through.
When warmer water and air temps permit, the shorty wetsuits (short sleeved, shorts bottom) are nice because they are easier to don and doff than full suits.
Surfer-oriented suits are stretchier than paddler-oriented suits, in my experience. They must use better neoprene and/or facing materials, though trying to sort through the plethora of proprietary names can be a turnoff. The fit will vary between makes and models, so try to buy one where you can actually try it on in the store first. If not, check out The Wetsuit Warehouse and Sierra Trading Post. The latter, in particular, has routinely had big discounts on some wetsuits. Just be aware that it's a mishmash of good stuff mixed in with crappy stuff.
From what you described (varying body mass), a drysuit would be easier to fit. You can vary the amounts of underlayers (which in this case ARE necessary) because they are cut baggy. Really baggy. But if budget doesn't allow it, that's the end of it, unless you find a good used drysuit for sale. It might be worth checking your local Craigslist and paddling-club classified ads for one before you buy a wetsuit.
Kayak Motor Kit
The Kayak Wing
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