What about a pool instead?
What I wonder is why people don't use a pool as the tank? It would not take much modification to add the filtering needed, and with a small greenhouse building over it, heating would not be extreme... Plus it would be easy to get in and out of for cleaning. The only problem would be you wouldn't have easy windows to look in, but so what? You could go swimming with the fish, or just look down in on them. Heck, with stuff these sizes (and costs) it wouldn't be too bad to add a couple underwater cameras to check the whole tank out.