I'm too cheap to send my calipers to Eric, so I've taken them apart myself. Everything actually looks good, except two things:

Apparently, the pistons have previously been removed by "that guy" (quote from Eric's video), so the lip that retains the dust boot is essentially gone. Is this going to be a big issue?

Click to view attachment

Secondly, the bore has some contamination/corrosion near the outer edge, that I couldn't remove with a scotch-bright pad. Do I need to clean this all off? Is so, how can I do this at home (I don't have a vibratory cleaner or anything like that)?

Click to view attachment

Thanks in advance!
Matt

Also, I broke the snap ring that holds on the spring for the handbrake lever. Does anybody know what size it is, so I can buy a new one?

Go buy a brake cilinder hone and clean the rust from inside the caliper.
You can Seddrak more attendance with sanding paper on the rusted edges where the seal sits.
It doesn't look to me that your piston is gone, just clean the rust and polish the outside a bit.
For the clip, just measure it's seat. I don't remember the size.

Freshly honed bores lead to fresh rust. Brake fluid is hygroscopic.

I "never" hone a bore anymore. It's the worst thing you can do to a caliper (but what do I know?)

Yes, you need to clean that off. Rust never sleeps. At the very "least" get the caliper bodies plated now. This is one of those things where being cheap will continue to cost you problems and money into the future.

The piston should be fine. We have all the parts.

Words of advice- just send to Eric and be done with it...