Replacing pads and rotors have nothing to do with brake fluid. If you wish to flush the brake fluid, there is a blue synthetic which is perfect and can be purchased locally at any auto parts store - DOT 4 compliant.

You need to measure the front rotors for thickness to determine if they can be reground to smooth. The rear do not take much abuse and can go many more miles than the fronts. If the front rotors have a large rusty lip on the outside edge, more than 1mm, I would consider getting new rotors.

http://volvospeed.com/850fbrake1.php

A simple 7mm allen tool is needed to loosen the caliper slides. I use a hammer against the tool to bang it loose. Same with the caliper bolts. The hardest part is putting the caliper spring on - I use channel lock pliers and a long screwdriver to get enough leverage. Don't forget a coat hanger to hold the caliper to the spring.

The rear brakes should get new springs, the old ones get rusty. The rear calipers don't need to come off, they are exactly like to 850s.

If you use FCPGroton it will take a long time to ship. How about http://www.autohausaz.com/
If you get OEM rear pads, they have a shim builtin.

Klaus