MMA is an umbrella term for a concept and combat sport involving various martial arts. The arts that are similar to MMA aren’t “MMA” styles: They’re hybrid styles, and they aren’t going to be about the combat sport more often than not. You’re going to have to check your local areas, and ask the MMA schools about what they’re teaching exactly. It varies, but rarely are schools going to flat out claim their own hybridization of styles.

The MMA school I attend teaches “American Tai Jutsu” (Striking- Kickboxing/American Kenpo, Grappling- Brazilian/Small Circle Jujutsu with some wrestling), but what I’m being taught is more than likely not going to be the same as what you’ll be taught. It might be full contact karate/combat submission wrestling, or boxing/bjj, or Kickboxing/wrestling. Who knows? You’ll have to find out.

Try starting with Judo.

There is no “best type” of MMA…MMA encompasses a huge variety of martial arts…Most commonly used is boxing…jiu jitsu…and muay thai. You have the boxing, now go to a MMA school and learn it. I’d start with jiu-jitsu since you already have some standup, that way you’ll be more of a well rounded fighter, right away.