Wheel bearings aren't that difficult to work on.... you just have to follow instructions. Anyone comfortable doing mechanical repairs on a car should be able to do bearings. Being a perfectionist certainly helps. The main thing is to have everything clean and contaminant-free. Replace bearings in complete sets (inner race, outer race, and rollers) if they are less than perfect. Mounting surfaces must be clean and free of particles and burrs for smooth and square engagement. Seals must be replaced at the same time.
I usually use mineral spirits or kerosene to clean bearings, then dry as much as possible. If you don't feel comfortable with that, you can follow up by cleaning any residue with alcohol.
I don't know why 90,000 miles is suggested. I'm still from the old school of regreasing every 30,000 miles. Maybe someone can shed some light on this.
95 E320 Cabriolet, 131K