This is my opinion, but I get the feeling most people dig deep into what they want the tank to look like, without really digging into the filtration needed to keep that system healthy and thriving.
Have you considered what type filter you want to use. HOB, Cannister, Sump? etc
I run a massive sump system on mine, Using a 125g 6ft tank as a Sump. Hit my Build Thread :)
The other thing is if you plan on keeping/growing coral is the Lights. Maybe the second most important investment you will make after buying the tank. I'm a big fan of Reefi-Lab Uno 2.0 lights. It's what I use, but by no means is that the only light available, there are a Ton, and some get really expensive. Mine are Very High End, but about half the cost of "Leading brands". Most High End lights have a 24 inch x 24 inch foot print. Means they will illuminate a 2 ft square area.
Bulkreefsupply.com has a really informative 52 weeks of reefing video series I highly recommend watching. Yes they try and sell you products they stock, it's a huge infomercial for BRS Products, but there is a ton of solid reef keeping info as well, just ignore their product ads.
And ask alot of questions here. There are a lot of folks here with solid advice on what works for them.
Good Luck.