Well first, this will not be a roaming network at all since the AirPort devices must be connected by a wired Ethernet connection to meet that criteria.
You will need to look at professional equipment (at professional prices) for the type of installation that you are considering. Unfortunately, we cannot help with this on this forum.
A good source would be Ubiquiti Networks, who will help you design a system for your needs. Good luck!
http://www.ubnt.com/airmax#nanobridgem