Hey Matt,
If you're going to be buying a lot of furniture or even want to upgrade stuff like carpet, flooring, appliances, etc, check out Direct Buy in Latham. Their url is directbuy.com . It costs a lot to join, but you can save a huge amount of money on this type of stuff.
As for the size of the house, if you think you might want to go bigger, maybe you could choose a lot and floor plan that would nicely accomodate additions. Just a thought
Good luck. I've been through the building process (although not a modular) so feel free to ask questions.
Oh ... I almost forgot the most important part.

It may sound silly, but consider the size and location of your tank throughout all of this. You may want to consider things like having a section of solid slab underneath your tank location and easy access to water supply and drainage. I'm not suggesting you design your house around your tank, but if you think you'll be sticking with this hobby and having a big tank, a little bit of insight can go a long way.