while changing my front brake pads I found 1 caliper piston boot badly worn and torn. Is the boot DIY replaceable and is it critical? Also, I was missing 1 caliper slide (small clip that sits under brake pad), is that critical?

 

The boot will keep dirt and dust from the piston seals. If dirt gets in there then the seal will start to leak brake fluid. I would either rebuild the caliper of get a new one.