Almost everyone says NOT to get a sniping platform for your first gun. I have no experience with them, but am inclined to take their word for it. I would recommend an AEG as well. I personally started with an MP5.
How much you should pay for your first gun depends on what you want. $250 could get you a good deal on the classifieds if you're lucky, they show up in that range sometimes.
Upgrade it to $400 and you'll be able to find a good AEG, probably with a battery and some mags and other things, if you're lucky, and probably full metal if that's what you're after (and why wouldn't it be?).
You're eighteen soon... wait for the classifieds and the retailers. If you MUST get an Aftermath for whatever reason, get a Kraken. I've never heard anyone say a single good thing about the Knight. Again, I'd still say wait for your AV tag and what comes with it; a full metal CYMA or DBoys is not out of your price range if you look, and it's not uncommon for higher end brands to show up at lower price ranges, either.
As Aoi said, you're in Toronto... you'll have no trouble finding an AV rep. There are a couple of retailers in the area who could do it for you, even...
Best of luck.