1. It would be better to have a like make/model, esp' if your current AP supports WDS or similar for setting up distributed wireless networks (so they act as one instead on fighting each other), but in theory providing you set them up with different channels (eg AP1 ch1, AP2 ch11 etc) then they should be ok. The problem would come when you sit in a place where the signal strength is similar from both AP's, you will start jumping from one to the other.
2. They don't have to be physically connected to each other in terms of a cable from one to the other, but they have to be connected to the same network in some respect either Wired or Wireless (Some AP's have more than one Aerial that can be set up differently, eg one for the Wireless Bridge and one for client access)
2a. If you cannot setup a bridge for any reason then a Powerline adapter for Ethernet over mains cabling may be an option, effectively turns your home mains electrics into a wired network. Then you can plug of a pair in near your existing AP and one downstairs to plug your new AP into.
3. As for No. of antenna/aerials etc, most 54Mbps have one antenna as that’s all they need, N Spec APs tend to have at least Three, and some (that appear to have non) have internal antenna.
WDS (Wireless Distribution System not Windows Deployment Service)
