Often, you can find sketches that people are giving away for free, that do ALMOST what you want. And modifying such a sketch Canberra much easier than writing it from scratch...
Fixed. I hate typing on my iPad.
How difficult would it be to have several remote battery powered sensors consisting of an XBee, Arduino Nano plus a sensor of some sort
Doable. Popular, even. There are some specialized Arduino clones that make it easier, like Funnel IO ( or JeeNode (not a zigbee, but similar) ([url=http://jeelabs.org/2010/12/18/rethinking-the-arduino-hardware-interface/]http://jeelabs.org/2010/12/18/rethinking-the-arduino-hardware-interface/)At this point in time, I'd be worrying about the choice of radio - Xbee seems is getting expensive compared to the low-cost BTLE and WiFi modules that have been coming out.
You do realize that custom software is often expensive, right?
How expensive, tens, hundreds, thousands?