Ok some disclosure I do work for Asus, in the wireless support division, but I'm gonna tell you this either way.
If you want to go consumer/soho grade and not park to large area specialty grade devices such as the EnGenius...
One router, whatever the best that you can get a hold of. - make that the main unit.
Then repeaters or repeater mode.
If you want the Asus way of doing it, look at an RT-N16 or a RT-N66U as the main, and RT-N10 or RT-N12 as your repeaters.
The 10's or 12's are both units with a dedicated repeater mode, and they simply act as a wireless client and repeat more wireless in said location with stronger signal.
The RT-N16 is a nice unit, no 5GHz bad which I doubt you need for this application, both have external antenna jacks if you want to get higher gain antennas or need them later.
16 is still being supported and has nice up to date firmware, most of the same features of the more expensive 66U.
Otherwise, DD-WRT, with a main unit, and then use repeater bridge mode on the subsequent units. They have a wiki article on how to set this up, follow it precisely.
I've done this on old WRT54GL Linksys boxes several times, works great. Repeater units will have half the bandwidth of the main unit for each leg. So if you have:
Main > Rep > Rep > Client the client will have 1/4th the wireless bandwidth of the main system.
http://www.dd-wrt.com/wiki/index.php/Repeater_BridgeDepending on the room shape, composition, what is in the way... etc, look at directional antennas or a mix of directional and omni to fill the space.
I've punched through a faraday cage like building before with 2 directionals, and thrown other signals up to 6 miles with directionals. That alone may make a
big difference in what you can do if something is blocking or too far away.