Hi Xavier,
I started investing in mtl 3 years ago and it's been going great. Will be looking for my 4th property in the fall.
Montreal is BOOMING. Vacancy on the island is under 2%! Do no get discouraged by the negative comments. When people say cashflow sucks in the big cities, it is a general statement. If you look hard, you will find great deals anywhere.
I recommend you start looking at Centris (realtor.ca for Quebec) everyday and study the market and prices in different neighborhoods. A lot of neighborhoods around downtown are being gentrified at the moment and are still very affordable (verdun, ville-emard, park-ex, HOMA). At some point, you will get a feel for deals and potential for optimization. Do no hesitate to message me if you have any questions I would be glad to help!