This is my first frag tank but here's my thought. Please correct anything that's wrong. Also this is kinda a budget build haha damn expensive hobby.
10 gallon sump (because I have one laying around)
With fuge section
Super skimmer 65 (because it's cheap and works good on my 29)
Koralia 550 because I have one here
Get a fluvial sea cp1
Not sure what size return pump to use
Also think about how much you want to spend on the different components.
This is a huge one. Honestly, I was just looking through some of your posts/threads/etc, and at the risk of sounding like a huge
edit, I wouldn't even set this tank up yet if I were you (unless it was going to be a total replacement for one of your current tanks). One of the biggest mistakes new reefers make is to go to fast. You've been in the hobby for a relatively short period of time, are still having quite a few new reefer/new tank issues, and having three totally independent systems is a LOT of maintenance.
If I was in your position, I would either dump one of your current tanks into this one and re-scape so it's something new for you, or stick this one in the closet until you really really really really know that you want to be doing three separate water changes, maintaining three separate sets of equipment, etc.
Also consider the question "do I really need a frag tank" before setting one up. Do you plan to sell your frags? Do you have anything in your display tanks that is so overgrown that it needs to be fragged? Do you have anything in your display tanks that is expensive enough to warrant the added cost of having a system to keep a separate piece of it (cost of electricity, salt, water, etc.)?
Edit: I just saw that your 27g doesn't even have water in it yet. Sell that thing and just set up this 30 as a display tank.