I can help you with some of your questions.
First your tank will hold about 460 gallons. The water weight alone will be about 3800 pounds!!! Given your tank dimensions, that will put about 180 pounds of pressure per square foot. Now, the live rock and sand will add even more weight as will the tank itself and stand.
With a tank this size I would STRONGLY suggest that you find someone familiar with structural engineering to look over your house to see if you can support something of this weight.
For filtration, I would go with a large (100 gallon or so) sump/refugium with a very heavy duty protein skimmer. I would go with the AquaC EV-2000. This retails for around $800. In addition, I would go with a DSB in both your tank and refugium to aid with denitrification. You will need around 300-400 pounds of live rock as well, either in your main tank or your sump or both. Macroalgae in the sump will help with nutrient export as well.
I am not familiar with mechanical and chemical filtration for a tank this big, but I would recommend both. Mechanical may be as simple as a fine mesh sock on your sump intake.
Definately plan on several bulkheads drilled on your tank for overflow and lots of water movement.
Sharks are VERY messy and dirty creatures that make lots of waste. You will need lots of heavy duty filtration to keep the tank clean, not to mention weekly 10% (~60 gallon) water changes. Water quality is key. Simple wet-dry filtration, ammonia towers, fluidized beds... will NOT do... chemical and/or biological scrubbers, extensive live sumps (refugiums), deep sand beds (DSBs)... and the like... of SIZE, capacity are required (unless you have access to clean seawater...) to maintain these fishes. Circulation and aeration need to be brisk, the water kept in motion for exercise, filtration purposes, and dissolved oxygen near saturation (about or greater than 7 ppm).
Personally I feel that even this system is too small for most all species of Leopard shark. You will not be able to keep one for more than a year before it gets too small. All sharks need ROOM... to move, grow, dilute their wastes, allow for gaseous diffusion of gasses for respiration... Not hundreds, but several hundred to thousands of gallons in the case of Leopards... Tanks of less than ten feet in length, five in diameter are equivalent to you or I being confined (permanently) to a closet... behaviorally, physiologically and psychologically prohibitive.
Plus, these animals need the water to be cold (50's to mid 60's being ideal) so you will need some serious chillers installed (run into the thousands for a system of this size).
If you have your heart set on having a shark, research some of the smaller species like the banded cat shark or the marbled bamboo.
Hope this helps!