(You forgot to tell us if you have any mates that already play, but I'll just assume you don't.)
Alternatively, you could use the teams as they are (before investing more money), and have a few games with your friends, to start learning how the game works, and to get to know the different playstyle of the teams you have from the box.
In no time you'll find that their skills and stats will induce you to play the game in quite different ways, and with that notion in mind, you'll look at the rulebook and notice there are many other teams with (almost) just as many ways to score and play/win your games.
By then you'll be able to do a more conscientious choice of either sticking with those two teams (and filling up the positionals that are missing) or have a go with a different team with a playstyle you feel that suits you better.
Oh, and welcome aboard (forum and game wise)!
