well, all that depends on the gametype... in Bank Robbery and Round team play, you can buy at the beginning of each round just by pressing the "buy key".. then a menu will show up, where you can buy, everything you want...like here:
in deathmatch and, we don't know yet.. it will be decided how deathmatch works in the next days/weeks... maybe we will post the concept in the news....
oh, and another thing bout the money system: you get more money for killing a better player than you than for killing someone who has less frags than you.. so that weaker players have more chances of getting kills...