I also agree about the Drysuit specialty course. Save your money. I read up on it, got some tips from my buddy's who already dive drysuits and just went for it. Everyone is different, but I was feeling pretty comfortable by about my 5th or 6th dive. Just takes practice
I didn't purchase any special undergarments. I've been using my polarmax thermals (that I already had) and they work fine. I found that after the water temp got down in the mid 40's that I would start to get cold after doing two dives. Not a big issue since most of us rarely do more than a couple of dives in a day when diving locally.
I remember diving with friends in late November here in Japan. Watching them shiver when the cold air hit wet skin, as they stripped off their 5mm semi-dry and 7mm wetsuits, brought back lots of memories I'd never go back to that