First of all, believe it or not mma is safer than soccer (football) The injuries that come with playing soccer are usually far worse than mma and as far as I know there is only one death in mma and quite a few in soccer throughout the years.
1. It depends on what you want. Learning a traditional martial art like karate or kung fu is great if you want some discipline and to really embrace the art in the word martial arts.
If you want to be as effective as possible in the stand up department I'd say learn muay thai. If you want to be as effective as possible in the ground game, I'd say learn Brazilian Jiu Jitsu. If you want to be as effective as possible in a real fight where everything goes, then jump straight into mma.
2. Diet, Diet, Diet. More important than anything else is what you put inside of your body. I could talk about diet but once I start I wont be able to stop so I'll spare my wrists for now.
Exercise wise, if you're looking for the most muscle growth that translates into practical striking power, I'd say stick to deadlifts and squats. Never ever do any isolation lifts if you're looking for practical size. For your ground game, nothing is better than weighted pull ups IMO. Once you put more muscle on your body, it gets easier to burn fat. Also, core work is also very important... you're going to need some strong abs if you want to do mma.
3. Start out with low rep deadlifts and squats at first to gain raw power and size. Once you have an ideal amount muscle that fits your frame well, I would start doing higher rep deadlifts and squats and I would also start incorporating circuit training. Strength over time is a lot more important than overall strength... remember that.
Since you're 5'10, I'd say a good walk around weight would be around a muscular 170-180 considering you said you had a bit of fat on you as well.
4.Stay motivated and hungry, my friend. Expect to get your ass kicked at first. It's a humbling process ;]