This is an excellent question and, as an old guy with decent experience and skills, I'll throw my $.02 in.
Of course, maintenance is more important than miles.
You gotta expect a few wrinkles, scars and scrapes. Anything worth riding is gonna show some wear. As long as she runs good, shifts easy, revs smoothly, and can get you stopped under control when you need it, that's what you need.
Try and make sure there is no crazy ex rider who hasn't gotten over it and is having a hard time letter her go. If you find any of his junk in her bags, just get rid of it. If you'd wanted brand new, never ridden, that comes with it's own set of problems in addition to higher cost, break in and that comfortable, well worn feel.
Try and make sure there are no hidden surprises either. Fluids should be pretty clean. Once you get your hands on her and get in there and feel around a little bit, there should be no crud or serious build up. Again, it is used and some goo is to be expected. If it's bad or if something smells fishy, pass. Could be a sign of more serious problems.
If she smokes little, I think that is OK. Too much is too much.
You don't want one that swills too much. That's usually a sign of hidden trouble. Moderate is fine and going heavy once in awhile when you're really kicking it out is OK, too.
At the end of the day, if it's a good, solid ride, consistently, and you're good to her and she's good to you, more often than not, it will be a happy ending for all parties and, new or used, isn't that the goal?
And, if she also likes to ride motorcycles, that's good, too.