Based on what you've just said, I would suggest to give some consideration to getting the Seaskin compressed neoprene suit instead of the membrane suit.
Get it with neoprene neck and wrist seals, zippered pockets, a pee valve, and the Kevlar knee pads.
Save yourself a bunch of money. For the water temps you're talking, you might not even need any undergarments, apart from shorts and a t-shirt. Maybe some basic long john underwear. Neoprene seals are much more robust than latex or silicone, so much less concern or need to have a replaceable system. And, the temps you're talking about really do not require dry gloves. Good wet gloves over neoprene wrist seals should do fine to keep your hands warm. You can possibly even get "wet" gloves that will mate up (just via overlapping sealing areas) with neoprene wrist seals so that they actually keep your hands dry.
With the trilam, in 15C water, you're probably going to want more serious undergarments, which adds to the overall cost as well.
If you get the trilam, you can definitely just have it with non-replaceable seals (neck and/or wrist) and have replaceable systems retro-fitted later, if you want. You can have the Si Tech wrist docking system and retrofit Kubi rings later, as well, if you want.
Personally, I have the Seaskin membrane suit and am seriously considering ordering the exact compressed neoprene suit I just described - for water temps just like what you're talking about. Very simple, robust, and convenient and only about 2/3 the price of my membrane suit. My only major hold-up is that I would REALLY prefer a front zip for self-donning. I really don't want to have to ask someone else to zip me up every single time I use the suit. But, for what it is and the price, I will probably go for it anyway.
