Personally I think you'll be disappointed with a single 15" sealed sub with a 300 watt amp in a 3500 sqft room. A ported sub would perform better in that range if you can only fit one.
Not sure where you are in Canada but if you can stretch your budget a bit this would be a decent option
Maybe you can talk him down a bit.
Another thought which I think tuxedo mentioned is...
Again, stretches the budget a bit but I think would offer more output than a sealed offering in the same price range.