Welcome,
I am in the same boat as you are. I have a 135g that I am going to be running a 55g sump on. It will be my first foray into building/plumbing/running a sump.
After getting advice on this forum, my plan is to run 2 pumps for redundancy. I am leaning toward 2x Jebao DCT 6000s, each running at about 70%. That way in the event that one dies, I can get by while waiting on a replacement.
If my napkin math is right, I should get around 950 gph total with them both at 70% at 5' of head, but I am sure I will tweaking the system so it runs quietly.
As far as the sump goes I plan on having 2x 7" x 16.5" Filter Socks in the first chamber, which I will rotate out weekly give out take. The second chamber I am going to run 6 gallons of K1. Third chamber will be empty to start with, but I eventually want some sort of Nitrate export (UAS algae scrubber or a bunch of Pothos, not sure yet, still doing homework on that part) and then the return pumps in the last section. Each chamber will be sectioned off with .22mm Acrylic. I will post some pictures as I go.
As far as plumbing goes, I am leaning toward a mix of PVC and vinyl. The advice I have gotten here is include as many unions as you can for maintenance sake, or if you need to replace/modify/move any part of the plumbing. So far on my drains I have unions coming pretty much straight off my bulkheads, and then a union ball valve going into each hole in my sump. I haven't started on the return plumbing yet,