Though I wouldnt recomend a Canadian Tire Bike for what youre looking to do, I find that you do need an indestructible bike to do some serious downhilling, which really is different from XCountry and technical MTBing. Just stick to that Kona. Nothing wrong with an $800 bike to start. But from experience, I had an $800 Trek steel frame when I started out, and it didnt last 2 seasons of the abuse so those who say anything will do aint exactly ridden it for the pure joy

Mind you it wasnt so much the frame, but the cheap ass components that broke and snapped. Components are huge in MTBing, and there is rationale to light weight - those who come downhill must first go up...and on technical shit uphill, or even easy uphills for beginners, you conserve more energy the lighter your bike is.
When you do get a bike hit Dagmar. Good beginners place, besides, its close to Toronto.