Most lucha business is done via walk-up, so its not like you need to get your tickets before hand.
Be careful would seem to good advice too, given the usual depiction of Mexico City and its crime issues.
Cubs likely has more tangible advice. I'm not familiar off the top of my head with any Mexican folks who post here, but there are a bunch who post at the DVDVR board who might be able to give first-hand suggestions.
If you're going those dates, you can catch the weekly Tuesday (7:30pm local time) show at Arena Coliseo. Of CMLL's DF shows, the Tuesday show usually ends up with the least prestigious lineup - you probably won't get Mistico, but you'll get a good main event.
There's a picture of the Arena Coliseo building up on luchawiki. A decent cab driver should know how to get there, but I've always been told that it's not a nice neighborhood - it's okay to go to, but just be aware of your surroundings.
When you get there, try going to the ticket counter first to purchase tickets. There may be scalpers around, but you're better off skipping them. Tickets are absurdly cheap compared to US shows - under $10 for front row some weeks. There's always plenty of tickets available before the show, so don't worry about buying in advance.
There are usually vendors outside the building selling masks and shirts and other trinkets, so buy before you go into the show.
There are surely other shows in Mexico City throughout the week, but they're smaller and not as advertised and randomly scheduled - pick up a lucha magazine when you get to Mexico City. The big show of the week is the Friday night one at Arena Mexico, but it looks like you'll just miss it.
For World Title, I'd say I consider the worst to be the SHORT term transitional champs. Kane, One night, beats Steve Austin in a First Blood match at a PPV(When Taker accidently clubbers Stone COld) and the next night losses it back.