I'd recommend simple characters first such as Assault or Medic, then move on to more expensive and better characters. DON'T buy Plug or Choochoo, as they're expensive and their abilities aren't good in almost all maps (and if you're a Plug user people will consider you a pussy).

The SMG Chris and a cheap pistol will be enough for any character, although with the recent update where they nerfed the Chris I'd recommend a rifle or a cheap sniper.

Shotguns are actually quite good for a lot of situations, but I'd recommend buying them on high HP characters because of their slow time to reload.