Trooper, there are a lot of questions there!
First off, undergarments don't have seals. Layering fleece UGs from other sports will work, but you will pay an excessive weight penalty for doing that (purpose-made diving undergarments are generally more compact and less buoyant than multiple layers of fleece from other sources). In addition, it is EXTREMELY important to be sure that whatever undergarments you are using will remain as solid insulation when they are wet, because if there is anything certain in life beyond death and taxes, it's that a dry suit diver will eventually end up wet.
As far as seals on the suit go, it IS nice to have easily user-replaceable ones, especially if you intend to use the suit for travel at all. The SiTech NeckTite system, with the related wrist rings, are a really neat way to have quickly replaceable seals. Using those rings, you can buy anybody's seals and use them, and seals aren't very expensive. If you go with the DUI ZipSeal system, you are constrained to buy ZipSeals, and they aren't cheap. If you decide to forego rings altogether, wrist seals are not a difficult user replacement item. Neck seals are a bit more of an issue, because you need the proper form to get the suit to lie flat for gluing.
What goes on your feet depends on the suit you buy. I am a fan of attached boots or TurboSoles; I don't like the hassle of a rock boot type setup, and I really don't like having something on my feet that I can kick off and lose (and I have).
If you are diving Southern California for the most part, you may or may not want dry gloves. I prefer them; they are both warmer and easier to get on and off than wet gloves. The dexterity is no better and, depending on the gloves and liners you use, may be worse. Dry gloves are like dry suits, though; eventually, they will be wet, and wet dry gloved hands are miserable. In addition, if you do any kind of dry glove system that eschews the inner seal, a wet glove means a flooded suit.
Fit in a dry suit is very important, particularly if you are buying a laminate suit. Although you can live with a torso that is a little generous, you cannot live with one which is too small, as it will affect your mobility and that can have safety implications. Legs which are way too baggy or too long can cause air-trapping and poor venting. It is worth trying on a stock suit, if you are thinking of buying one from any manufacturer, or being extremely careful about both vendor and measurer for any custom suit. My custom suit nightmare is one of the big reasons why I dive a Whites Fusion, which is one of the only suits out there where fit isn't terribly important at all.