As JJ has mentioned, the thing to be prepared for are the costs. Everyday I am surprised by yet another unexpected cost on my tank build. For instance, today I am spending $200 for extensions from my lights to my ballasts, since the standard connectors aren't long enough when put on a big tank. There are so many little things like this, that you will be surprised no matter how well you plan.
I don't know how much a plywood tank costs to build (my tank cost about 1/10 of the system price) but for all the other stuff I would think that a 600 gallon reef for 10 grand would be very cheap. 15 to 20 would allow for a bit more redundancy and a few nice features that you will want in the long run. Of course some people would spend 40+ on a 600 gallon tank. So, you can do a big tank on the cheap, just be prepared for what cheap means when it comes to a 600 gallon monster.
As for the humidity issues, an HRV alone will gaurantee a cooked tank come summer. With that much water and 400 watt halides, you almost have to have a chiller.
Brad