Hey there, welcome to the site. I'd welcome you to the sport, however there are a few things you should know.
You got it right that you can't import guns from the states, no matter how much of a better deal you think you'll get, the odds of getting raped by customs are pretty good, so IMO it's not worth trying.
Even if you did have a large private field to play on, it's not recommended that you go and have skirmishes with your friend. There are a few reasons for that, such as safety and liability. You don't want some passer-by to see you with a realistic looking firearm and call the cops. You also wouldn't want anyone getting hurt and then holding you responsible. Legit fields have insurance specifically for that, so it's a better idea to let them deal with that headache
As for playing airsoft at the age of 15, the short answer is that you won't be able to play at any legit fields till you're 18. Some places with the proper insurance and whatnot make exceptions and allow 16 year olds to play, but that's not something you should expect. You'll need to prove yourself to be responsible and gain some respect in the community. What you can do though, is you can go to an airsoft event and talk to people, ask questions and prepare yourself for when you'll be able to play. If the community sees you're a responsible person, that's when exceptions may be made for you.
If you want your dad to buy an airsoft gun for you, that's entirely up to you. You should know though, that for you to use it legally your dad would have to be there with you when you use it.
I don't know much about airsoft in the Toronto area, but that's the general low down. Get in touch with some local players, and get known. As a final piece of advice, read, then read again, then read some more, THEN ask a question.
Cheers and Good luck