Regarding pad thickness, I just use the pad backing as a guide--when the front pads are down to about 3/4 the thickness of the backing, replace. I let my rears get down to about 1/16" before replacing since they don't wear very fast. And on front brakes, always replace the rotor and pads as a set and you'll avoid a lot of problems. I use Zimmerman rotors and PBR Metal Master pads--no dust, no squeal.

The caliper bolts can be tough--remember they get very hot. I had to use an 18" breaker bar on mine. If that doesn't work on yours, you may need to buy an impact wrench (you can get electric ones) or give up and take it to a shop.

While you are in there, a good time to flush your brake fluid--should be done every two years as it is hydroscopic and that water buildup causes rust which is a prime cause for caliper failure. Be very careful loosening the nipples, they will break if your wrench puts much side force on them and THAT will definitely complicate your day.

96 850T 106K miles. Bilstein HD, IPD swaybars