If I was to start a project such as this, I would have a look at
Kip's 1020g DIY tank build...
Before starting, make sure your house can support the "few" pounds of moist sand/EE! its going to put a lot of weight on your flooring, so it might be worth getting an engineer to inspect your house! And if you house isn't up to supporting such a tank - aka, don't place the crabitat upstairs!
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- then you might need additional support.
Remember whatever you build with, it has to be hermie safe and pretty much hermie proof! Probably to biggest cost would be to gather all the materials: a good quality pond liner isn't going to be cheap, and I imagine making a structurally sound enclosure would also take a bit out of the bank account! With some careful shopping, I imagine that it may set you back around $500, without interior furnishing!
In such a large tank the overall temperature and humidity should be relatively stable. But a nice ceramic heat bulb - a couple of the 100watt would be handy, even 200watt if you live in a colder area - and a nice running water feature, humidifier, moss pit and properly moist substrate should keep the humidity at the right level.
Hopefully this will become reality, I really enjoy living through large builds such as what your envisioning! Keep us posted (please!).