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Version upgrade availability ... age old question and virtually impossible to answer. The best advice I can give you is to buy from a well known manufacturer like Asus, Toshiba, Samsung, etc. If you go for a budget pad, there's a much greater chance that it will be orphaned at an earlier version of Android.
Official updates will come from the manufacturer, but it's usually possible to flash a rom with a later version. Keep in mind doing so will void your warranty and they won't support it.
Android 2.2 and 2.3 is perfectly capable of doing those things you list as well as much more, however the 2.x versions are optimized for phones while the 3.x versions are optimized for tablets. Of course, the next version of Android, (Ice Cream Sandwich) is supposed to be for both, but we won't know until it's released, most likely early next year.
Only you can judge the technical savvy of grandma. Can she use a VCR? Does she own a computer? If so, she probably can handle a tablet. However, if she still refers to rotary dial telephones as "newfangled things" then probably not.