bikemig 

You should be concerned about the cantilever brakes reaching. This is a known problem with touring bikes built around 27 inch wheels. The difference in diameter is only 4 mm but many cantilever brakes lack the necessary adjustment.

The rear spacing is not a big deal if going from 120 mm to 126 mm. The frame can be sold set to 126. You can also just push open the rear triangle to get the rear wheel to fit. This will be tough the first time but get easier. There will be posters here who don't like this approach but I've done it few times and as far as I can tell, it works just fine.

It's tough to beat these 27 inch wheels from velomine for $115, Sun CR18 27 5,6,7 Speed Freewheel hubs Road Bike Wheelset [72274726665] - $115.00 Velomine.com : Worldwide Bicycle Shop, fixed gear track bike wheelsets campagnolo super record vintage bike

These wheels will let you run a 6 or 7 speed freewheel. Shimano still sells a pretty good 7 speed freewheels in a touring size (14-28). 27 inch tires are readily available from panaracer, continental, schwalbe, etc.