Your choice will depend on what lens or lenses you intend to carry. If you are going for a 35mm only, a serviced M2 or M4P/M4-2 will work well. If you are carrying a small complement of Rf lenses, say 28/50/75 or 90 - a M6 or M4P is a good choice. If the meter dies in a M6 it just converts it into a temporary M4P anyway.
There are few things to worry about with the M's. Finders and shutters are well proven and tend to work for a long time. Biggest problem with the shutters is inactivity - thats when they start acting up - slow speeds go REALLY slow and the 1/500 and 1/1000 either start capping or simple go way off.
The M2/3/4 and M4P are easy to adjust rangefinders on. Just a small jewellers screw driver will do. The M6 and later require special tools.
In 50 + years of using M's I have had a handful "fatal" crashes with them, but considering that I probably have owned in excess of 150-175 M's of various models that is not a bad track record. IF a camera is banged sufficiently hard, things will go wrong, but truth be told, that can happen to any equipment.
M2's are simple, basic cameras with few things going wrong. Finders and rangefinders are easily adjusted, shutters pretty well bullet proof - apart from burning holes in the curtains if you are not careful. If you decide on a M2 (or M3 for that matter), carry at least one extra take-up spool, just in case. The chances of delaminating a finder is slim - it really has to get "whacked" to do that. The M2's finder share most of its part with later cameras and can be fixed. The M3 has a unique finder and repairs are costly.
The M4 and later has the "tulip" loading system, no extra spool required - but it has a rewind crank that is exposed and easily bent out of shape. It is more common on the M4-2 and M4P as the material is softer on those, but M4's have been known to bend and jam too.
The top plate issue is not anything to worry about, brass or zinc would easily survive either way. It has more to do with long time heavy use and wear.
Obviously the M will be used for a particular type of shooting, "walk about", hanging out in bars and street shooting. This is a case were less is more. A single lens, fairly fast and moderate wide angle will probably suffice ,Summicron 35, pre asph Summilux as it is considerably smaller and cheaper than the Summilux Asph, Nokton 35f1.4 or my personal favorite for that kind of shooting, the Nokton 40f1.4. The last one works remarkably well as a 35 and/or 50 and with f1.4 is fast enough for most everything. The advantage of a moderate wide is also that if the camera would suffer a catastrophic failure and the finder is crushed - you can "guestifocus" with a 35/40. That is getting tough with a 50 and longer.
Whichever body you pick, have it checked out prior to going and shoot a fair bit with whatever lens and film you pick. This is just so that operating the camera becomes 2nd nature.
As you are probably going to be shooting "blind" ( not see the processed film until you get home) - I would recommend that every 7-10 days you pick up a short roll (12/24 exp)l of color negative film, run it through the camera at all speeds and run it through a 1 hour lab. It will give you an idea of the "status" of the camera. If by any chance a shutter start acting up, say capping at 1/1000 - you then will know that and simply avoid that speed.
Don't be paranoid about the camera - it will probably come through fine. The main thing is to enjoy the trip.
It could be that the M will come through unscathed, whilst the SLR's system, which is more fragile will be the one that will give you problem. Zooms are easy to knock out of alignment and the zoom action tend to 'suck" up dust and moisture!!!!