That was helpful guys.. Here's some more info.
I'm still in the planning stage, really, so anything is open to change. The existing deck ended up costing the seller of the house, so I'm aware how that works

It appears that when he put the deck on, he didn't put flashing against the house and just tacked the deck onto the outside of the house *through the plastic siding* so there's water seeping inbetween the deck and the wood of the house.
Heating: I wasn't planning on heating the enclosed part specifically, rather I was just going to insulate it with standard fiberglas insulation and big insulated windows. I would love to get a little outdoor portable wood stove in there, but I don't think it's possible if it's enclosed. Can you get one of those things on a deck if there are bricks under and around it or would that be asking for trouble?
Skilliz: I'm pretty handy, and I can read instructions and take suggestions and apply them well. I know some other people like that as well, so I don't think skills will be in short supply.
Materials: What's the benefit of having a cedar deck? The smell? I could make a little cubbyhole for my cigars

I was just planning on using the standard run-of-the-mill pressure-treated wood that everyone else uses for decks, but only because I didn't know there was anything else (except for that plastic recycled milk bottle stuff). If there's anything better out there though, I'd like to know about it..
Size/Construction: The size I gave was purely an estimate of the current deck and how I'd like to make it bigger, which is to say, I would like to make it run alongside the house a little more, even extend to the master bedroom and add sliding doors in there. That just might be a pipe dream though. My house is a split-entry, but the bottom floor only goes about 4 feet into the ground. The existing deck comes off the second floor kitchen area, and I can walk underneath the deck without (almost) bumping my head.
Thanks again! I appreciate the help!
-sab