1. When I was fitted, there were a lot of measurements takes, and compared to some tables. I am 6'2", 220lb, 46"chest, 36" waist and an 18.5" neck. My suit is 3XL and the undergarment ix XXL. Normally, I wear XXL for shirts, etc. The drysuit is the only thing I own in 3XL! Everyone will be a bit different for sizing though, depending on the sizing tables...
2. If not winter diving or ice diving, I usually wear a thin Merino blend base layer that I picked up at Costco a couple of years ago. It was cheap and does the job nicely. I have also been known to wear a t-shirt and shorts under the drysuit, but long garments are more comfortable if there is any squeeze. On very warm days with cool water like we have here right now, I stick with a t-shirt and shorts. In cold weather/very cold water, I go with the undergarment that came with the suit. Basically... I choose my undergarments based on the weather and diving conditions to be sure I am comfortable and will not be sweating excessively inside the suit. I am rarely, if ever cold though, so everyone will be different.
3. I am still using the original neoprene neck seal (not easily user-replaceable) with no issues. I had my drygloves installed for me. Not sure how much trouble a DIY job would be. Replacing the Si-Tech wrist seals is straightforward now that the rest of the install is done. Until I had the wrist seals replaced for the drygloves, they were not user-replaceable.
4. It is crushed neoprene, so it is buoyant. Diving the drysuit, in fresh water, with an AL80 and minimal undergarment, I use 24lbs right now.
5. My socks depend on the dive. In warmer weather, I use plain athletic socks. No real insulating ability there, but I don't need it when I am wearing them. When ice diving, I usually have up to two pairs of wool socks on for the added insulation.
If the water is particularly cool, I also use Fourth Element Xerotherm wrist warmers under the dry gloves, and in particularly cold water, have Fourth Element neoprene glove liners for inside the drygloves as well.