You might be able to get away with using your 105 derailer on a 32 if it works on a 28. I might try it out first before buying a mountain or whatever derailer. As Sheldon says, derailer specs tend to be on the conservative side and you can often push them a little farther than the manufacturer recommends with no ill effects.

As for question 2, http://bicycletutor.com/calculate-chain-length/