Since all wireless radio's transmitter power is regulated by the FCC, distance is relative. A hundred feet between walls/ceilings? Nope. The D-link DIR-xxx series is about the best. My DIR-622 manages my entire one story home.

A combo unit is available but I'd never buy one. Get a modem and a router. You'll only need a switch if you have more than four wired devices.

A wireless router will handle around 255 wireless devices. Around. And as you add devices the bandwidth is shared and thus, the speed will drop if multiple devices are "surfing" at the same time.

Then again, if you have basic DSL you'll be limping if more than one or two devices tries to connect. Hopefully you have cable.