First Bike Shoes

Depends on your budget and personal usage. Mountain bike shoes are fine for a beginner, even on a road bike, they're a little heavy, maybe a little harder to get into, but may be more comfortable and are generally easier to walk in. With MTB shoes you may be limited as to which brand of pedals you can use, for example I have yet to see a MTB shoe that will accomodate a Look cleat.

Generally speaking, road shoes are lighter and have a stiffer sole for better energy transfer.

For faster transitons, especially if you're eventually going to try the shoes-on-the-bike method in transition, you may want to look at triathlon shoes, which give you a lot easier entry and exit to facilitate the process. 

Your best bet is to visit your LBS and try on several models to see what fits and feels the besst to you.

Pick a quality, service-oriented vendor. trisports.com and all3sports.com have good reputations for service, performance.com is spotty but you may get a good service rep. Key is to get someone you can talk to on the phone and explain your situation and sizing and get viable recommendations. Sidi, for example, is one of the few that makes shoes in a wide size. A good CS rep should be able to put you in the right shoe.

Probably most important is to understand their return policy. I wouldn't buy from someone online if my risk extends beyond the cost of shipping to return the goods for a full return, no questions asked. (Except for the rare ebay auction that is too good a bargain to pass by).