I know it can get very cold in Ottawa in the winter. Are automobile engine block heaters common/necessary? If I move a car from the US, would it need to be retrofitted?
There is no perfectly correct answer to this question. Block heaters aren't uncommon in Ottawa, but lots of cars don't have them and get along okay.
The last new car manual I read recommended that if you have a block heater you should use it only if the temperature was going to be below -18C, suggesting that they are 100% certain that a properly working car will start at temperatures above that. According to this Ottawa averages less than 20 days per year where the low temperature reaches -20C or below, so following the recommendation would mean you won't get a lot of use out of the block heater. My experience is that if you use synthetic oil and don't try to get more than a few years out of your battery you'll have little to worry about above -30C with a modern car, and in Ottawa it is rarely that cold. On the other hand, if you do see a <-30C morning you may feel very smart for having bought the block heater.
I think you wouldn't regret buying the block heater (and if you move somewhere further north block heaters become standard equipment) but if you don't buy it you'll rarely have difficulty in Ottawa since it isn't that cold. You sort of need to decide for yourself whether you prefer to feel perfectly safe that your car will start under all conditions or would be happier not spending the money.
Do buy snow tires, though.