By setting up a sump, you have as much invested as you would with my suggestion. Although the initial cost of my proposed filtration scheme may seem High to many, the coast is justified when water purity is the goal. "Layered" filtration is the way to go. You can clean on element of the system without having to worry about causing "mini-cycles" in your tank. Much less stress on you fish and less death. Add to this the water flow and oxygenation of the water is very high. In a big tank, that is key. As I see it, you can do it now, or you'll be doing it later.
Having a large tank is more than having the large water capacity. The most important part of any aquarium is the filtration. Whether it be a small 5g tank or a massive 720g, the key to keeping it healthy is the filter system you choose. It protects you investment. To say that you don't want to make that king of investment now is ludicrous. Down the road you will be spending much more than your initial cost just replacing fish. Not to mention the time you will be investing with maintenance by slacking on the filtration now.
Water changes will still need to be done weekly. But, you will have an easier time with gravel sweeps and other tedious chores. The Filstar XP3 costs around $175-$200 each, depending upon your lfs. For that price you could almost have the system I suggested, around $400-$450, and you'd have an an awesome filtration system. It would be "layered" versus a single element system. So the investment is well worth it. Not only does it cost about the same, my suggestion is a much better way to go. You are getting a 4x better system for approximately the same amount of money.
Allow me to break it down here.
(2)Magnum 350 canisters ----------- $159.98
(2)Emperor 400 power filters ---------- 89.98
(2)Marineland 1140 power heads ---- 41.98
(2)Reverse flow kits --------------------- 10.90
(2)Fluval 3+ internal power filters ---- 75.98
(1)undergravel gravel filter ------------ 28.99
Total ------------------------------------- $341.81*
*Does not include the price of the "eggcrate" light diffuser which should cost about $10 at most home improvement stores.
The cost of (2)Filstar XP3 canister filters --$338.98
Note: all prices are from petsolutions.com which includes free shipping on orders over $199.
So, I hope I've broken it down enough for you to understand and broken down the myth of cost. As you can see, just because a filtration system is sounding complicated, it is not any more expensive.
An additional perk to reverse flow filtration. Due to its nature, pumping water up through the gravel instead of pulling it down, the substrate stays cleaner(fewer gravel sweeps) and the bio-bed has a more even flow of clean oxygenated water. This results in a cleaner healthier tank. By it pumping water "up", it forces any particulates up into the water column to be filtered out by the other "systems".