I did a similar project using Jeenodes. Wireless and Arduino all in one. http://moderndevice.com/product/jeenode-v6-kit/
Dead simple to use. http://electricprojects.wordpress.com/2014/01/05/jeenodes-weather/
Right now, mine just goes to an LCD screen, but there is a Jeenode on a USB stick so you can get the data into your computer.
I have my outdoor sensors in a plastic box inside a Stevenson Shield - http://en.wikipedia.org/wiki/Stevenson_screen
The inside LCD screen & Jeenode are in an old cigar box.
Looks like you have done some research. I'm not sure how a WiFi shield and an Xbee would work on one Arduino.
Make sure to check for any pin conflict on all the items you want to add. I would start with sensors first and get them to work, then work on getting the LCD screen to work next, then worry about other stuff later.