As far as all copper, that's good. Better thermal conduction between the core and the fins = better cooling. Only disadvantage is it would weigh slightly more.
Regarding NAPA - ask them to let you look through their heater core book. Try to find one that you like for a very popular/common car. Larger volume tends to equal lower price. In comparison to a lot of cars, Caddies aren't common.
Again - look in the yellow pages for radiator repair shops local to you

I would think in Toronto there would be a fair number. Oh, and do you have any AutoZone or AutoShack type stores up there (NAPA tends to be a bit more expensive for some things).