hey Jake, it all depends.... I have had my current setup running for about 1.5 years so its balanced itself out after the first 10 months of operation. I don't use carbon on a normal basis and i think (IN MY OPINION) that its completely unnecessary except for certain situations... Carbon is for pulling things out of the water, and sadly it doesn't know if those things are GOOD or BAD things... also once the carbons full it can leach toxins back into the water. I know i try hard to keep all those things in my water. I dont use carbon and i have no issues and have NEVER used it in my saltwater tanks.
with that being said there are a couple times you will want carbon. I have a carbon reactor i made with a RO canister i paid $20 for and hooked it up to a maxi jet 1200 pretty easily. I plugged it into a Christmas tree remote control for the lights i picked up for $5 at biglots. anytime I'm adding corals, fragging corals, or my leathers are shedding, i will turn it on for the day only. this pulls out the bad stuff without affecting to good stuff the other 29 days of the month...
FYI, that RO cannister i used for a carbon reactor can be built the same was for a phosphate reactor, just put some phosguard or GFO in it!!!
some corals are prone to feuding with each other so some people will run small bags to help with fighting corals but salt water doesn't require it. if you are using RO water and a good quality salt, not reef crystals.. Aqua Vitro salt is my jam... and 20% water changes every two week to replenish trace elements that carbon will pull out and you'll be good. Id spend the money on a protein skimmer and phosphate reactor.. that's ALL you need. you dont need C02, you don't need a Calcium reactor if you do your normal maintenance and use good salt..
Also one final word since your building your setup now. i always run a protein skimmer that is undersized on purpose... i spend allot on high quality coral and fish food and i dont want my skimmer stripping my water of it... i need that stuff floating around so i don't have to target feed, never have, never will target feed, only for fun and showing off to non reefers while their ogling my tank, my duncans love shrimp...lol
I run a coralife 65 gallon skimmer on a 75 gallon tank that has a 38 gallon sump, 20 gallon fuge, 40 gallon frag tank, and a 20 gallon hospital quarantine tank that has valves that can be removed from system... so that's 195 gallons of water total!! my corals and setup thrives like this and greatly benefits from the extra nutrients in the water. my tank is clean and spotless because i have all of the right cleaning crew fish and corals along with a ton of fuge algae and live rock, yet my water is technically dirty even though you cant see it is.. that a good thing though. what my reef and fish don't consume, the cleanup crew, algae in the fuge, and the phosphate reactor will keep it balanced..
good luck and spend the money right the first time so you dont go backwards..