I purchased a custom built tank through FTD. The 15' x 4' x 4' tank was built to my specifications. I recommend anyone looking for a large tank to consider FTD.
Your "HEAVY" stocked comment is concerning to me. I can't help but picture a battle with high nitrates. A continuous drip system would eliminate frequent water changes if you have high nitrates. "HEAVY" stocked also conjures up thoughts of frequent changing of filter socks or filter floss in a sump. If you want to avoid that, consider one or two of the smallest Ultima IIs (Ultima II-1000). The easy, painless backwashing of the Ultima II quickly removes the poop and debris from the filter. Since the purging is done with tank water, besides cleaning the filter, the first part of your water change is done and all you have to do is refill the tank. it would be helpful to have a drain close to the Ultima iI.