Most of what I would do if starting over was stated, but here's my list.
1. Reasearch all equipment prior to purchase.
2. Go as big as possible for the physical environment in which your tank will be located.
3. Make a custom stand, from whatever material you choose, to allow for the largest sump possible. Use either magnetic doors, or a large single sliding door for access to the sump to keep maintenance easy.
4. Line the bottom of the stand with seam welded acrylic to contain spills and any potential flooding and use an Apex with the water alarm if you do flood so you don't cause damage to the house.
5. Prior to getting anything, determine what livestock you plan to keep such that you design the system for its intended inhabitants. For me....this item on the list is big. I recently took several months preparing my system for fish that I always wanted but at a noob, didn't set my system up for it.
6. Design the sump as big as possible to allow for expansion, additional equipment, and ease of cleaning. Again, this was a big item for me.
7. Acid wash dry rock and seed with a small piece of LR from a trusted local reefer.
8. Don't put sand or LR rubble in the sump. It's a pain to clean and leads to disasterous problems down the road.
9. This is contentious among reefers, but I would design the sump to allow for multiple filter socks that can easily be swapped and cleaned and keep at least one set of extra socks on hand plus one additional.
10. Keep a section of the stand dry for use as an electrical cabinet for all the necessary power and control cables..
11. Cable management system for the power and control cables in the dry section of the stand.
I'm planning and upgrade now and the size will not be much greater than I have now but it will be as big as I can go given the environment. The above are keys to my upgrade planning.