If your not doing a custom tank then a 180g tank is probably the way to go, a 125 is nice but short front to back and the 180 will be better for aquascaping. For a sump I would just go with a rubermaid trough, I think they make one 100g or so about 4' by 2', made of polyethylene it's indestructible and out of sight so looks isn't a huge deal. Typicaly youll see sumps in the 30-50% of the tank size, and only about 50% full. The bigger your tank is the less important sump size is because you have the stability in the tank, but bigger is always better if money isn't an issue. For your pump you want to be pushing 10x tank volume max more like 6-8, it will be reduced to around half after head loss, plumbing and bio buildup in the first 6 months. PVC should match pump or be bigger. For a skimmer you have a unique opportunity by not having a height limit so get the tallest skimmer you can find that makes sence. Another thing you could do is place second cryptic sump or simple resivoir under the display that flows to the main sump basicaly just additional water volume to increase stability.