Is it feasible to use a single Arduino Uno microcontroller for wireless communication with sensors, and also host a web-server using Ethernet shield? or is it better to use a separate microcontroller for web-server ?
You have not told us what form of wireless comms is required by the sensors.
It may be worth considering a Mega because it has 4 hardware serial ports.
On the surface a Yun is attractive because has WiFi and Ethernet as standard, but it does not have a spare hardware serial port.
You can, of course, use SoftwareSerial on an Uno or Yun, but it is not nearly as good as hardware serial.