The Gazelle Champion Mondial is a really nicely made bike and can be picked up for much less than many other bikes of similar quality. There are different versions (AA, AB, cyclocross etc etc). There are other Gazelle road bike models, but the CM is probably their nicest.
Here is a good source of info: http://jvs.webklik.nl/page/homepage
For your questions:
1) Probably not: the largest spacing for 1970s models seems to have been 126mm, suitable for 6 or 7 speed. Maybe you could squeeze a 130mm wheel into a 126mm frame.
Earlier models (1971-5) were smaller still, 120mm for 5 speed. You would have to have the frame cold set to take a modern wheel.
2) Champion Mondials usually used Reynolds 531 tubes which usually take a 27.2 seatpost, but there seem to be exceptions, so take care to check when buying.
3) Should be a standard British threaded BB so fairly easy to replace. Removing the fixed BB cup is usually the biggest problem but a good local bike shop should help.
4) Can't really think of anything beyond the standard stuff you would want to look out for on a steel frame (rust, dents, straight forks etc). I don't think these frames were faked or counterfeited very often. The CMs usually had very nice paint, often metallic, so anything with bad paint might be suspect.