Warning:
Long post filled with a lot of ideas/advice.
FWIW, I'd also suggest running the sumps in parallel rather than in series. Lower chance for failure, and some sort of minor clog/messup is inevitable at some point, so I'd also suggest making the pipe connecting them as large as possible.
Also, it might be worth putting emergency drains on one or all of the sumps. Just an open bulkhead connected to a drain pipe near the top of the sumps, above where water would end up in the event of a worst case scenario.
For the sumps themselves, I'd suggest going cheapo given they don't have to look pretty and you're running a business (and thus minimize costs). Extremely large stock tanks are very inexpensive on a gallon per gallon basis, and much less fragile than glass in the event of inevitable kicks, dropped rocks, etc. Building a sump out of heavy duty pond liner in any sort container reinforced by a strong wood or metal frame also works well.
Also, I'd strongly suggest having very easy to use auto-WC and ATO systems. A large (multi hundred gallon) stock tank for SW mixing with a gravity drain unto the system is ideal. Alternatively, you can use an external pump on a closed loop as a mixing pump, with a 3 way ball valve to divert flow to the systems after mixing is complete. Having another 3 way ball valve to choose whether to move the mixed SW to a fish or an invert system is also a good idea, as it means you only need on mixing tank and mixing pump, again saving money. If you have an emergency drain on one or more sump, and a gravity drained mixing tank, doing a WC is as easy as opening the valve on the mixing tank. That saves a TON of man hours in the long term, and thus a ton of money, as having an employee spend a long period of time performing a WC is basically just paying them by the hour to watch water drain, and possibly diverting them away from customers in the process. A gravity drain rather than pumps for WC also makes it true "fire and forget", with no need to remember to turn off an expensive pump before it burns out.
You should also look into the price of bulk purchasing pre made salt mix vs the price of buying the chemicals needed to create it in bulk.
Also, consider how you want to do in-tank furniture and ask around. Using plastic or sintered glass "rocks" makes life easier if you ever need to dose copper or sterilize. White HPDE bottoms for some tanks are also worth considering. They look much better than bare glass and are easier to keep clean than sand.
For the tanks themselves, I'd suggest consulting with any reasonably local builders to see if you can work something out. You might be able to get all the tanks built to spec for far less (and potentially at higher quality) than a mass manufacturer. You can have the custom builder put "Tanks Made By" stickers/etching on everything, giving them free advertising and getting you a connection with a custom tank builder. That way, when a customer asks about a custom tank, you have a solid connection to refer them to, and having them subtly advertised throughout the store increases the odds of them asking about it.
Usually, aquariums are low gross margin (sale price minus wholesale cost) items, and they are extremely high overhead. They take up a lot of space, have a good chance of being broken before sale, and do nothing to draw customers or encourage any sort of minor impulse purchase, unlike high quality livestock or show displays.
It's also worth trying to work out similar deals for other initial fixed costs, like lighting. It might be worth talking to someone like buildmyled.com to see if they can give you extremely good pricing to light your tanks in exchange for free advertising and a retail distributor.
Those initial fixed costs are big, and minimizing them (without halfassing stuff the matters) means you can get into the black much sooner, and afford to spend more on the cool/shiny stuff like high quality finishings/paint, better display tanks, etc. Setting things up to minimize long term labor for basic maintenance also means that you can afford to pay your employees a bit more, and hopefully attract better ones in the process.
Just my two dollars worth of advice!
