floor joists split

I'm currently in the process of digging out a basement under a 24x36 house. I wouldn't be doing it if I didn't have to. The main support beam and a number of the floor joist's are pretty well shot. The house is 50 years old and there is only about 12-16 inches of crawl space. I've owned the house for about three years, and after running a phone line to the computer about two years ago, I noticed the problems. The folks I bought the house from used 2x6's to strengthen the floor joists, and they aren't working. They didn't even attach them to the floor joists, there's about a 1 1/2 inch gap between the floor joists and the scabs. That's one problem, the other is that I want to replace the old galvanized plumbing with plastic, and possibly rewire. Since I'm not a little guy, I figured that digging a basement is my only option. I'm digging in hard red clay, any suggestions on how I should install some drainage and waterproofing. I would also like to reduce the number of supports under the main beam. Would steel I beams, wood I beams or laminate beams would offer the greatest strength? If it's possible, I would like to have only one support in the middle. the span would be approximately 15-16 feet.

I've read reference to a vapor barrier on numerous occassions when finishing a basement. Not to sound ignorant but what exactly is it & what's its purpose? Does one need it? I recently framed two basement walls(Interior non-load bearing partition walls) & assume I don't need it for them but I'm considering covering an outside poured concrete wall & from what I've read in the forums it might be needed there.