Mark:
I'm completely in agreement with Dave. It sound like you guys will be moving. Building a layout with that complexity, helix, hidden staging etc., would not be prudent. HOWEVER, if you are going to build a sectional layout, which sounds very reasonable and logical in your circumstances, better to PLAN from square 1. It's more work, effort and money to build in sections but very wise if you are moving. You may want to rethink the entire plan including putting all the track on one level. It's not as interesting but it would be a hell of a lot easier to move and reassemble.