You can visit Canada for six months a year without a problem.
If you want to stay permanently, you need to apply to become a permanent resident. If you have children in Canada, you may be able to be sponsored. There is no special retirement category, so you'll need to qualify in some category intended primarily for people of working age. There are various categories in which you can apply: skilled worker, investor, etc. Some provinces also have their own immigration programs. The criteria for these will vary from province to province. Some programs require you to live in that province for a period of time (such as if you promise to start a business), while others only require you to intend live there, and you can actually leave for another province after a few months.
Usually being older will count against you. However, under some programs, you may be able to make up for this through education, work experience, etc.
If you have $800,000 to invest and a net worth of at least $1,600,000, you may well qualify under some provincial or federal investor program.
Here are some pages with details of business/investor programs:
http://www.canadavisa.com/quebec-business-immigration-investor.html
http://www.canadavisa.com/pnp-business-immigration.html
Here's some additional information:
http://www.livingabroadincanada.com/getting-started/retirement/#.UhAvlNAfLWU
http://www.coalharbourcondos.info/non-residents.html (See the comments by the immigration lawyer.)