Every mechanical camera needs servicing. Whatever state your used camera is in, send it to be serviced by a 1st tier leica repair person. The LHSA has a list of reputable people. The amount of fraud present on the CLA front cannot be overestimated. Contrary to reports, I have never experienced a "true" CLA costing less than USD 250-400, i.e. restoration of the camera to factory spec. The same things applies for any used lens. Before you start using it send it in and have it serviced.
Get Günther Osterloh's Leica M book. Read it through 2 - 3 times or more if necessary. It will explain the majority of things you should know before you even start. Leica M's are unforgiving of user's lack of knowledge.
On pristine mint cameras the bayonet lock spring may not function perfectly. This can have the consequence of your lens popping out while focusing. Any retardation in the bayonet spring lock movement or tension must be tended to ...
Don't leave your camera in an uprotected state for any length of time. Buy yourself a Zing bag large enough to have the camera in at all times, no matter whether it is in or out of your camera bag. Anything falling on the M's will invariably hit the lens or one of the other small glass surfaces the camera has.
If you don't like a neck strap (and they often are a bad idea, because you will invariable sway the camera into something and break something) get a wrist strap that secures the camera at all times. Don't let other people play with the camera without the strap.
The half cases do reduce wear. Mr Zhou makes a terrific half-case for a modest amount.
The first thing that gets lost are the ridiculously expensive sun shades and caps. Either get used to guarding them fanatically, or reserach their generic equivalents.
If you can afford get things insured by a professional camera insurer (they insure against sloppiness). Gear that gets used will break and/or be stolen/lost. Best to be prepared than be sorry.
More generically, if you are unfamiliar with analog photography, get a good light meter and learn how to use it (not self-evident and not covered by its owners manual; several good books are available on the topic). If you want to be a fanatic (sometimes not the worst thing) get your light meter adjusted, the gold standard is George Milton of Quality Light Metric in California.
It is important to think about the analog work flow (where and who to get your film from, and film developed, printing, etc. done) all of these factors may a big difference in the quality of the outcomes. The APUG forum has resources on this topic.
Most importantly, you can enjoy the fact that you can use your Leica (with the obvious caveat of the existence of film) forever. You will get better with it every year, and you will never have to replace it, and there is nothing better. That is relaxing.
