I'd really wait to buy the expansion, adelgary.
There's
more than enough content available in the stock game to keep you occupied, and most of the added content for CoP won't be low-level stuff (really kicks in around 30, as Alex said). The storyline doesn't kick in until Rank 6, I believe, which is quite a ways in.
And besides, there's always that chance that you simply won't enjoy the game, and blowing extra money on an expansion just to find that out would suck.