Hi,
I'm in a similar position (6'5", 225lbs). In response to your questions:
1) Yoga. Yes,it will be weird and yes, your freinds will take the p1ss. But I think it's the best/quickest/safest way to a stronger core. Your posture, flexibility, core strength, breathing and tree hugging ability will all improve. I prefer the slower, more relaxing type of yoga. I have a great 70s book (Richard Hittleman's 28 Day Plan or some such - it's on amazon). Give it a go.
2) Yes. Just be careful in making sure you get a bike that fits. This is where a good bike shop will show itself. Tell them what your aims are and what you want.
3) No idea. Sorry.
4) There's hundreds of saddle threads. It will take while to get used to and find one you like, but you will.
Check out the Clydesdale forum for motivation and tips.
Good luck,
Tom