I've acquired 4 used machines. The machine itself will be just fine with no particular cleaning. There's really no way to clean the machine, and no real need to.
Remember that the air mostly goes from the machine to the patient. While some air goes up the hose when you exhale, it's "swimming upstream" and has a long way to go to get to the blower unit.
Hoses, masks, and maybe the water tank need to be cleaned or replaced.
Change the filter and give the machine a cosmetic cleaning. The machine may have odors, especially if it was owned by a smoker. I got one machine with sort of a vaporub smell, but it wasn't bad. If necessary, let it air out for a while. You might want to let it run for a while. Other than cigarette smoke, it will probably air out pretty quickly. Also, once you start it and let it blow air for a few seconds, the flow of fresh air should flush out a lot of the smell.
If you're paranoid about germs, run it for a few minutes to dry out thoroughly. Do this every day for a week or two. Few serous germs will live for very long in a fairly clean and dry environment for long.
When you buy a used machine, you'll often get masks and hoses as well. Those are a little more likely to be a germ problem. Most mask parts without foam will survive being washed in the dishwasher and tossed into a big pot of boiling water, but turn the heat off before putting the mask in.
Hoses, including "short hoses" that are part of the mask, are more heat sensitive, and should be treated more carefully. 30 minutes soak at 70C is more common for sanitizing a hose. Hoses are a lot harder to clean in a dishwasher because water doesn't get into the inside unless you take special measures. You can rinse most hoses really well by hooking them to a kitchen faucet.
Heated hoses may or may not take immersion well. Check the manufacturer's recommendations.
There is also Control III disinfectant for soaking CPAP equipment (not the CPAP machine or humidifier.) You could soak the hose and tank, thought. Supplier #1
sells it. I haven't used it. I dishwash and boil.
Most current water tanks can be disassembled dishwashed as well. I've dunked Respironics and S9 ResMed tanks into the boiling water as well. If you ruin it, they're not that expensive to replace.
Whatever you do to clean, it's good to dry thoroughly and let it sit clean and dry for a week or so. Blow the hose out by hooking it to a CPAP machine with no water for 30 minutes or so.