In terms of costs for a SW
tank, it varies on area and what all you end up doing. You have the tank, so that is one thing down.
Do you need a sump? No...but more water is always a good idea.
How much live rock? 1-1.5 lbs per gallon. You don't have to get live rock, base/macro/dry rock works just as well so you can save some money that way.
What powerheads? Koralia's are quite popular due to not being as expensive as some. I have several in my tank and like them, even the ones that sometimes start in the opposite direction. There is a fix for it though.
The key to saltwater is keeping water pristine. Either buy ro
water from your LFS
or buy a unit yourself. BRS sells nice units and you can get them off ebay pretty cheap. I have a 5 stage unit that works well. The other bit is lighting. Corals need lots of light, depending on the type of coral you want. This is where the cost is. My suggestion would be to look at the taotronics LEDs from ebay/amazon. This is as cheap as keeping any photosynthetic coral your heart desires gets. Just try to get a unit you can dim so you can acclimate coral to your new, very powerful lighting.