Barring no issues like cordless phone interference I guess all depends on how you want to proceed. As far as our friends adding the old Linksys in the role of AP severed them well, they wanted a simply solution and didn't want to make a project out of it.
If you dont want an off-the-shelf out of box AP and would like to do a the dd-wrt method you probably only need an N router. Again if you have no environmental issues impacting your wireless you could get away with just about any AP or dd-wrt compatible device. You could probably pick up a new older
Buffalo router with dd-wrt already installed from the factory for about $55 shipped if that's how you want to go. I wouldnt pay any more than that and if you shop around you may do better. Then all you do it configure it as an
Access Point and assign it an IP address outside of the ActionTecs dhcp scope so when the VZ router reboots your router you dont get any dhcp conflicts.
Here we are running in mixed mode with G/N and I have no plans for 5 GHz at the moment. IIRC the 5 GHz is the one that really struggles with the walls & obstacles as you mention but is great with NO obstacles, Im certain I be corrected if that is not factual.