want to start my first SW tank

i am going to start my fist SW tank this summer.
i currently have a 55g semi-planted (but in need of re-scaping and maybe a new substrate) with 3 angels, 2 german blue rams and 2 cories, i need more lol

anyways its down to either a 24g nano cube, which are awesome and ive been reading a lot on the mods and whatnot, or a 20g L or H.

where is the best place to buy a nano cube for the best price? i can get them at one of my LFS for really cheap, used though.

i dont think im going to do corals yet, but i want a FOWLR at least. ill probably get about 20 lbs live rock.

any suggestions would be helpful with some links etc.
thanks!
-adam

alright, well i decided to buy the 20L tank. now im buying my stuff online.

if i want to do a FOWLR, i am going to buy an aquaclear filter. my question is do i want an AC30, or do i want a bigger one to make it into a refugium? or would the power filter itself with no skimmer be ok?

i dont know if you need those in a tank with no corals, like i said, ive never had a marine tank before.

i also want to know what heater is good to buy as well as a powerhead. i am also going to buy a powerhead for my FW tank (which im thinking about taking the plants out and just putting the fake ones back in. IDK though)

EDIT: i also want to know how much substrate to buy as well
thanks