There's usually more than one way of doing a job. You just need to dive in and figure out which way will work best for you.
You definitely do not need to drain all the coolant - just the tank. I replaced the sensor also.
I had a look from underneath, but the only thing I did under there was remove and replace the overflow tube. The rest I did from up top but, YMMV. I did not remove the braces and plate.
Here is Pedro's DIY:
http://www.pedrosgarage.com/Site_3/Replace_Coolant_Reservoir.html