Here is my advice, for what it is worth.
First off I'm a TMA guy, just so you know where I'm coming from. And many people will probably say my advice is horrible.
Okay. From what I've seen of MMA classes and MT classes (and I've actually taken some of both) is that they are very good and teach good stuff. Where they seem to fall short, however, is actually taking a ton of interest in the beginner unless he shows some early promise in the ring. The training can be very inconsistent (as you described).
They have A LOT to offer the guy with some experience under his belt. While I don't have the time to really train seriously in MMA I think I'd learn a lot very quickly if I did. The reason I don't is the same reason your dad got out of karate.
But for a beginner there is nothing wrong with a karate class/judo class. In most traditional martial arts class the philosophy is: everything I need to know as a black belt I learned as a white belt. So they really focus on fundamentals as break them down (whether it is a round house kick, a throw, a pin, whatever).
So my advice would be to take a karate type class and a judo class. Enjoy them for a couple of years. Then if you really want to take MMA you can jump in...and while things might seem strange at first I think people from TMAs seem to catch on fast.
Case in point. When I did the MMA my brother was in the class with me. He had a very good wrestling background and for him it was like a duck to water (the grappling portion). The standup, on the other hand, was fairly easy for me to get. The guy who had really good standup in the class, as a matter of fact, was a Kenpo blackbelt. That guy used to knock my brother on his ass when we did standup, but we'd pretty much be even (the Kenpo guy and myself). Grappling, however, my brother schooled us both....
There is nothing wrong with MMA. It is good stuff. But it is a lot for a beginner to digest. If you're young I'd consider judo/karate at this point.