I don't think I have ever bought a complete flipper rebuild kit from PBR or PBL. But as you can see from the PBR Williams kit above why lots of people swear by it. It is complete and includes everything you need including an allen key too! I would say the only thing missing in this kit is a flipper gaping tool but if you've rebuilt enough flippers in the past you can normally do this by feel or in a crunch I heard people use a business card.
I usually keep a healthy stock of common flipper parts such as coil stops, coil sleeves, plunger and crank assemblies for both sides.
I have a few EOS switches on hand but I think in the past 5 years I've only had to replace 2-3 of them on my games. Otherwise I just clean the contacts and re-gap while in the switch edge test to make sure it engages at the end of stroke. If you notice when removing the old coil sleeve that it is very tight or seized it could be a sign that the old EOS switch is bad or wasn't gapped properly causing the coil to overheat swell up.
I don't always change the playfield bushings whenever I do a rebuild but I do have a few on hand just in case I notice there is a crack or wear. If you notice after rebuilding your flipper that it still isn't strong to make shots it could also mean there is too much play in your old playfield bushing causing flipper drag. This in turn will cause your playfield to look like over time:
The other parts which come in the PBR kit such as return springs, screws, nuts and washers I have in my general hardware stock. The only screw hardware that has ever broken on me during removal are the little silver ones which are used to hold the playfield bushing into the mounting plate.
Lastly if you notice that after you have rebuilt your flipper but they still look uneven like this:
Then you probably need to replace the small black rubber grommet which has worn down from the the plunger linkage hitting it.