You say 125-175, so I'm gonna guess your looking at 2strokes? Air cooling is not bad, just not as efficient as water cooling, and if you are looking at 2strokes, they have all been water cooled since about 1982. 175cc bikes haven't been produced by the big four(kawi, suzuki, honda, yamaha) since the late 70s/early 80s, so if you go that route, it will be air cooled.
If you are in the market for a four stroke, unless you get a fairly new bike, it'll be air cooled.
There is no such thing as an "easier to fix" bike; every company has engineered something poorly at one time or another. Personally, I
hondas.
If you are starting out in the sport, try for an early to mid 90s bike if you've got the coin. Typically less problems in newer units, and parts are more readily available.
As far as trail breakdowns go, I have never had one, except for a flat tire. You should keep the thing in good enough condition to avoid this; you don't wanna be 10miles back on a trail and have to push your bike back to the truck.
