I wouldn't even buy an external drive packaged and sold as such. You typically get a better / faster / cheaper (in many cases) drive if you build it yourself.
I'd go with a 300-500 GB backup drive minimium. An image backup of a typical OS may be in the range of 60GB, a backup of her data will likely be at least tens of additional GB.
It is HIGHLY recommended that the drive be capable of storing multiple independent full backups so that if she finds that her most recent one is somehow corrupt or incomplete perhaps her one from a few days prior to that will be OK.
In the US 500GB goes for about $89, 300GBs are often on sale for $35-$60, and drives in the 640-750 range sometimes around $89-$99, and 1TB around $115.
Seagate Barracuda 7200.11 ST3500320AS 500GB 7200 RPM SATA 3.0Gb/s Hard Drive - OEM
http://www.newegg.com/Product/...x?Item=N82E16822148288
Western Digital Caviar SE16 WD6400AAKS 640GB 7200 RPM 16MB Cache SATA 3.0Gb/s Hard Drive
http://www.newegg.com/Product/...2136218&Tpk=WD6400AAKS
Something like one of those would be quite suitable.
To interface it to USB as an external drive, put it in an enclosure with a FAN (important)
such as:
http://www.byteccusa.com/produ...747sata/ME-747sata.htm
http://www.directron.com/st0010u.html
or others... it doesn't really matter too much as long as it matches the drive type inside (SATA and SATA internally or PATA and PATA internally) and has USB 2.0 outside. Don't even think of getting one without a fan though.
You'll have a much higher performance (faster backups / restores) drive than many of the made to to external units, a generally comparable price, and typically a better selection of enclosures (i.e. one with a fan to prevent drive overheating).
Sometimes you get a better warranty on the drive also, seagate individually sold drives often are 5 year limited warranty items, whereas I've seen external enclosure products from various sources with as little as a 1 year warranty or less.
Canada stores? I hear NCIX is popular. I don't know about others. I suppose the shipping rates from somewhere in the US like newegg or directron.com or svc.com may or may not be prohibitively expensive.