First things first: The frame and fork will need brazed-on cantilever brake bosses. In other words, the bike needs to be made for cantilever brakes -- you can't just add 'em. (Not without some brazing skill and equipment, which most of us don't have.)

They stop just fine if you've got 'em adjusted well, but that's the trick. Some people find adjusting them to be trickier than other types of brakes like calipers or linear pull v-brakes.

Mounting hardware, like I mentioned above, are studs brazed on to your frame and fork. Each brake arm slides onto that stud, then you just bolt it into place.

Front and rear units are the same.

Hopefully, wedged/curved washers will make sense when you've got them in your hands. Those shaped washers allow you to set the pads to your desired angle, both up/down and toe-in.

The more you know about cantilever brakes, the less of a gamble it is. If you've worked with them, you can probably eyeball the auction to make sure the necessary parts are included. Or if they're not included, you can figure out if they're available. For example, some brakes have a proprietary straddle cable or external springs. If those items are missing and have been out of production for a long time, you might have a hard time finding replacement parts.

One last point, which is something I just recently learned: You'll find that older bikes, like mid-1980s and earlier, may have cantilever brake bosses spaced more narrowly than newer bikes. Because of that, a number of new brakes won't work on those old frames with narrowly spaced bosses. Old brakes might be easier to get working on newer frames with wider bosses since you can just adjust where the post is clamped into the brake arm. But even that could be problematic if the arc of the pad contacts something like maybe a wide tire. To avoid this problem, I'd suggest trying to stay with brakes that are appropriate for the period of the frame & fork.