Hey bro.. yes you can get into good shape with your friend's help and probably be able to do amateur MMA competitions. Usually they have pankration style rules which means open palm strikes to the face but everything else is okay. Some you have to use headgear, some don't. It's really up to who ever is promoting the event which could be an MMA gym or private promotion. You don't even need a lot of experience for these cuz they are like Boxing/Kickboxing smokers, you just show up, sign up and get your mouth piece ready. If you're in good shape and half way decent you should do okay. With the whole BJJ / MT thing.. the top tier talent usually have BJJ/ Wrestling/ Judo/ Sambo or MT as their base style.. which they've been doing for years and they start to add stuff from other styles. They have been competing in their style for years, and so once they step into the cage they aren't shook up by first time performance nerves. You don't have to be great at any one thing to be good, but you'll end up probably having to take the road in learning one of those styles to really improve. As for taking it into a real cage fight like KOTC or UFC.. then for sure you can't just train in a garage or Golds gym, you have to be with a legit MMA team/gym cuz you need sparring partners who can make you better. Iron sharpens iron bro. Check out this link for listings of events and just check one out near you. Good luck with training bro!
http://www.fightresource.com/viewlistings.php?cid=4#