Well first I'd invest in a game I'm sure to enjoy and has clear cut rules on boxing. Would really suck if you got all set up and started boxing that game only to find yourself banned later eh?

Guess it depends on what you're interested in.

Look at your big picture. If you go with WOW you can get regular WOW without the expansions for 5-15$ each (they run their 5$ special sometimes). Because it's going to take you some time to get 5 toons from 1-60 and maybe by then you'll have the $$ for the TBCX5 and continue on,

In the long run it is more expensive because you can buy the Battlechest for 30$ which includes vanilla WOW+TBC (you save 15$ from buying them seperately.)

Guess I'm trying to say no need to buy all the games + expansions if you're starting from scratch. Buy just what you need and add to it later.