Not really. That is, an acoustic guitar is not any more of a "beginner's" instrument than an electric guitar is. It might be cheaper, and it may be quieter (or possibly not: you can practice an...
No, they're all the same. Each string is tuned in the same manner. However, it may be found to be more difficult to play certain chords on an acoustic than an electric due to neck differences, but...
Martin are probably the best acoustic brand of guitar, although, they do cost between $4,500-$10,000. If you are just starting out, or have been playing for a year or so, I would recommend a Yamaha....