If you want good wifi coverage in all areas of your home especially if talking N or AC which is on 5ghz and much shorter range your going to want multiple AP.
I would look into unifi AP, they are very reasonable in price start at $89 for new gen AC models 2x2 and pro 3x2 goes for $149. They are poe which allows for easy mounting in the proper locations. Which would be your ceilings or wall.. Not some router sitting somewhere on a shelf or on the floor under a table, etc.
I have a much smaller house and have 3 of them. Just 1 floor. While you can talk to the AP on the other end of the house - it does not provide for the BEST speed, especially if your going to have some clients close and others far with much lower rates. I have 1 in the kitchen near the patio door, and max out my internet without any issues out on the patio 75/10 with AC devices.
I have another in the middle of the house hallway and then one in the guest bedroom on the far end of the house. There is no place in the house that you do not have FULL signal.. And I don't have to share bandwidth with the devices that are on the other end of the house, etc.
To proper do wifi in anything the smallest of open areas your going to want multiple AP.. Especially going to the new AC stuff.. Wait til 802.11ad hits your going to really need an AP in every room.
How your still using G with that many devices is just nuts.. Talk about SLOW as shit.. What is your internet speed?
https://www.ubnt.com/unifi/unifi-ac/