I have wanted a Pacman frog for a while now, decided to buy one shortly, but need some help please.

I understand there is one of these frogs that is harder to keep than others, is this right, if so, which one is it??
I am thinking i will get a green one, (Ceratophrys cranwelli) will this be ok for a complete bigginer, i did keep 2 european green toads, but some time ago.
I have found a place to get it from, they are tiny, about 2cm, just under 1 inch long, seems very small but will this be good??
My tank is 20inches X 12 X 12, is this size ok as a starter tank??

Thanks for any help.

Marg.