I've been using drysuits for years for winter dinghy sailing in a fast, wet, non-too-stable boat - can highly recommend them!
I'd have thought a sailing drysuit would be fine. They usually have wear patches on the seat, knees, etc, and are not too thick. I did my PB2 in my sailing drysuit, my mate with me was lent a very thick diving suit and he melted in it. So I'd suggest getting a sailing version, but get it "roomy" so you can layer up the fleeces underneath for when it's cold.
If you're going to be spending hours in it, I'd highly recommend a breathable material - then of course breathable fleeces work best underneath (like comfy skiing kit)
Attached dry socks keep the tootsies lovely and warm when you have to wade into the nasty cold stuff. The trick to getting drysuit socks into boots is either sprinkle the boots with talc first, or if you're not too bothered about looking flash, slip a thin plastic bag over your foot before you put the boots on...
My suit has latex seals, the wife and kids have neoprene - neoprene seems a lot more durable and if you're not planning on spending hours in the water not being quite as watertight won't matter
Be aware the necks come tight and you may need to cut it down carefully and progressively until it is still snug but you can breathe and the blood can reach your head
Generally it's one of those "stretch for a good one and it'll keep you warm and dry for years" things. I think mine was £300 and something for a good breathable Musto suit, though, don't see you'd need to go much more than that..
Hope this helps