I have a cheap Atlas TC411 tire changer and a Snap on Manual spin balancer. I only use them for personal use but I have several vehicles, so I change out several sets of tires each year - but I'm not using it every day and it's not a business use machine. As far as I know it will do 20 inch wheels fine.
Many years ago, I had a job in a gas station changing out tires on one of the old style coats air machine that was a chore and things always got dinged up - the new ones are way better than those old things, and they make it pretty easy to change a tire - I'm so glad I didn't buy one of those No mar or other manual changers.
The used tire changers I looked at all work pretty much the same. There's a bead breaker on the side which is driven by a large diameter air cylinder under the machine. There's a clamp system that uses shop air to clamp the jaws on the rim of the wheel, there's a motor that spins the entire clamp plate assembly and there is a mount/demount head that swings into place over the rim and has a heavy extending bar that the mount/demount head is attached to. The head swings into place, the operator lowers the head to the wheel, and when you lock it down it moves away from the wheel slightly.
That's the basic machine and all versions seem to have the same capabilities. Any quality differences in the brands are going to be the quality of the switches and controls and mechanisms and bearings that do those basic functions. My cheap machine has plenty of power and capacity to do any consumer-grade tire/wheel, and I would think it would work fine for a small shop.
The only problem I've had with my machine is that the guy that bought it new didn't have dry shop air, and the bead breaker cylinder got water and rust in it and then the piston hung up on the rust. I was able to fix it pretty easily, but it shows the importance of dry air.
There are several options you can get on the tire machine that drives the price up. Devices that lift the tire up to the machine; lever arms that hold the bead down on low profile tires, top side bead breakers, and such. I saw a video of a european machine that robotically does everything for you.
The problem with these automations is that it adds complexity and cost to a relatively simple and quick to use device. It would certainly speed the job up, but mounting/demounting is about the quickest and easiest part of the job of changing a tire. Getting the tire up on the machine can be a bit of a chore, but the actual demount and mount is simple once it's on the machine.
I've changed even very stiff sidewall 50 profile tires on my machine with hardly any sweat and none of those extra arms . You will need a couple of plastic bead clamps - but little blocks of 2x2 wood cut into cubes worked fine for me the one time I could have used an assist arm. .
The one option that I would consider mandatory is a bead blaster. I had to buy a cheetah because my machine doesn't have one.
As far as spin balancers, I learned something about them too. basically all spin balancers work exactly the same way too. Again, there's a wide range of automations that your spin balancer can have. Basically the machine needs the wheel mounted correctly and needs three pieces of information in order to balance a wheel. It needs the wheel diameter (or the diameter that the weights will be placed). It needs a reference dimension from the machine to the closest edge of the wheel (where the inside weights will be placed), and finally in needs a rim width in order to calculate where the outside weights will be placed. The machine then calculates where to put weights and shows you the exact location in order to balance the assembly. If you want to just use stick on weights, then you just adjust the rim width to the inside of the wheel face and it will tell you to put a bit more weight on, but the balance will still be correct.
So the balancer can be completely automated - where it has probes that extend and measure those dimensions, and spin the wheel up and turn it to where you need to attach the weight - all automatically, or it can have the operator do all those things manually - like my snap on hand spin machine.
Again, those bells and whistles cost more money and will eventually need expensive repairs - but do they balance tires better - no they don't. They balance tires exactly the same.
In fact, my balancer has dials that you input the dimensions on, where most newer ones have digital inputs and I think the old style was better. I can input the dimensions in 10 seconds on my machine - no scrolling through menus, typing on some cheapo chiclet keyboard or inputting the wrong data - spin the dial to a rim size, done. Theres a little extending ruler to measure distance to wheel, then spin the dial. Measure the rim width with some rim calipers, spin the dial, done.
If I were doing a lot of tires, I'd probably opt for an automatic spin motor, but I'd be pretty selective beyond that.
The real trick on balancing tires is your carefulness, speed and accuracy of mounting the wheel to the balancer. You want good accurate holding tools and you want a speed nut which can save many seconds in mounting the wheel and removing it. The other pricey option that you'll want to consider is a lug mount fixture for the balancer. Some cars use the lug nuts to center the wheel, rather than the center hole, and you have to have a separate fixture that can balance those type wheels. Many universal aftermarket wheels use lug nut mounting because they are made for many different wheels.
One of the reasons I have my own balancer is that I can get a wheel much closer to perfect than the guys at the tire store ever do. It takes me a few minutes longer, but it feels much better to drive the car.
