It's great to see a sketch on paper. I absolutely agree with the bigger, the better. Your needs are very likely to change, & building your own provides great flexibility to adapt.
When we started our large system, we planned two tanks. It was a 220 display & we used a standard 75gal sump. We paid $100 for x6 1/4" baffles & $90 for the tank.
Here's how our sump build changed over the last 18 months...
We did a bunch of reading & asked a lot of questions then created a plan:
We designed 4 chambers with intakes on both ends flowing towards the return chamber in the middle.
Next bought the supplies & got to work:
The "finished" product for what we needed at the time:
Then plumbing & cycling:
The algae scrubber was an unexpected addition, but turned out to fit reasonably well in the return chamber:
But that wasn't enough, so after we upgraded the display to 300gal, we kept building to solve new problems (dedicated 50gal refugium tank & 18gal surge tank):
The 75gal remains a cornerstone of the system. We cut out the last baffle reducing to 3 chambers providing a larger return chamber which was necessary for the surge device. If we lose power, the 75gal is pretty full, but it has the capacity.
Hope this helps. Enjoy your build.