The first year I was diving I extended my season here in Ohio, by doing the following......
1) Suit up in a heated building
2) get out of the suit on sits, get to a heated building, and put on a stocking hat.
3) Bring a cooler of hot water, or a thermos of hot water to pour inside all neoprene before the second and third dive. Warm neoprene is much more comfortable that COLD neoprene.
4) (I never actually did this, but I KNOW it would help ton) seal all glove to suit and suit to boot seams with duct tape. Then fill the suit with warm or hot water. Then dunk your hood into hot water.
5) Don't make the wrong decision like I did that first year. I did the third dive, and after a hour ride home in a warm vehicle, in warm clothes,I was still so cold I could barey walk up the three steps to get into my house.
Hypothermia is a cumulative process, don't push it.
6) Although I now dive dry with my DUI, If I were today looking to buy a drysuit, I would be sending a PM to Bob 3 from this board, and ordering a Viking extreme... Don't worry about the undergarmet, use several pairs of decent long underwear, you can worry about the undergarmet later.
HTH,
Dave