I doubt a actual invasion of Canada and it's not the Winter that would push it off.
The major threat to Canada today is the issues in the far north regarding border disputes of territorial waters with a nation whose winters go to -60 before windchill.
It's that no one would gain anything but trouble from it. due to the fact that they would be picking a fight with the second largest military in the Americas followed by First. Because if anyone did invade the Canadian land mass The US would be backing Canada actively.
but still a nation needs some degree of defence and a Coast guard is not defence.
On top of which Canada is active in NATO and Joint operations with the US.
Now onto the Fires, none of the Boats Canada has were new they were all Victoria class and bought used. The newest of the boats launched 25 years ago.
If Canada was to buy new subs I would say 1) this would likely be some time in the next decade, 2) Being Blunt Canada is about as likely to buy Nuclear subs as I am at Dating Scarlett Johansson.
This means the UK is out. the UK has phased out it's diesel submarine's All the current boats of the UK are Nuclear. The US is limited. There is now 1 Diesel offering an Unmanned Sub by Boeing this Echo Voyager this actually might be an option for buy though as unmanned boats could extend the mission capabilities keep prices down due to smaller size and patrol for longer times.
So What is a realistic option? for say the late 2020's?
German Type 214
French Shortfin Barracuda
Spanish S-80 class
Japanese Soryu class
Swedish A26 submarine concept