First thing is that radios like the UV82 come with random frequencies programmed into them from the factory. Not only would it be illegal to transmit on most of those frequencies, it would potentially interfere with public service or other licenced business users. This means you must reprogram them using a programming cable plus software from either Chirp or Baofeng.
The UV82 is approved by the FCC for Part 90 (business) use, as well as amateur (ham) bands. It is primarily an amateur radio, and is not designed as a family or unlicenced business “walkie talkie” type radio. It is also a very poor scanner, assuming you could even find frequencies to scan. Most public service radios in Canada are on a trunking network and cannot be scanned by radios such as this. Plus, virtually all police, fire and EMS radios in Canada are now, or will soon be, fully encrypted, and cannot be monitored by any scanner.
So basically, you DO need a licence to transmit on business bands in Canada, or to transmit on amateur bands (which are the only two bands you can legally use this radio.) Interference with licenced users is actually taken pretty seriously in Canada and the U.S.
Note that Canada does NOT allow MURS radios, so you cannot program them for the U.S. MURS frequencies or you will again be interfering with licenced business users.
In Canada, you can use both GMRS radios (under 2 watts) and FRS radios without a licence needed. When the U.S. changed rules for FRS to expand them to 2 watts maximum, they actually helped match to existing Canadian law. So you can use GMRS radios purchased in Canada if they are under 2 watts, plus FRS radios purchased in the U.S. or Canada because they are manufactured as being under 2 watts.
There are some really good quality FRS radios now hitting the market, and this is what I suggest you look at instead of a re-purposed amateur radio like the UV82. Check with our forum hosts at buytwowayradios, and they ship quickly to Canada.
As for your other questions, range is very dependant on antenna height (because UHF and VHF radios are all line-of-sight), receiver quality and how you are holding the radio. It is less dependant on output power. A 5-watt radio will give you better range than a 2-watt radio, but not by much. The actual range in real-world conditions for any UHF or VHF radio is not much more than a couple of blocks in an urban environment. You MIGHT get as much as one mile in a rural area with no hills and few building or vegetation between them.
As for changing antennas for better performance, I can guarantee you will be disappointed. I have tested nearly every aftermarket antenna on the market, and most give the same or worse results than the factory antenna. If you need max range, take it off your belt, away from your body (which is an excellent absorber of RF radiation by the way) and get the antenna as high as possible.
Technically, the UV82 can be programmed for GMRS and FRS frequencies, and few people will know (or care.) But you are just not going to get better performance than some of the really good FRS and GMRS radios now hitting the market.