u dont need two overflows but u can do a center one if u want depends on what ur lookin for...
u can do two smaller corner ones or a larger center overflow up to u id rather do a center one its less chance for leaking and less plumming IMO
well if u wanna keep sps its a hole nother monster
ur guna need a nice size skimmer, something like mine im running an ASM-G4 skimmer in my sump
ur guna need lots of flow on the order of 25X's plus depending either by utilizing a closed loop system with some sea swirls or what i have an Oceans Motion 4 way they allow for more of a random flow pattern than just a straight return from the pump
ur guna need some good lighting at least some 250W MH's id say on that tank do 3 if ur electric bill isnt crazy like here in so cal.... i only run 2 due to my electic bill already costing me an extra $100 a month for my tank the way it is now another MH would increase it by another 30 a month! but all my sps, clams, softies, lps and anything else is doing just fine but i run DE's
id say if u want some sps and clams ur guna need a calcium reactor too a dual chambered one is better i have the MRC-CR2 works great i never dose anything for calcium or alk
id say id invest in a UV filter too thats more up to u but i think they help
ur prob guna need a chiller too
there lots of things lol take it one at a time learn what u want and go from there
u have any questions ask here i spent many hours researching and i have a similar tank to what ur wanting so feel free to ask ive got some good opinions once in a while lol