When I first started reading your post, the Vaya was the bike that immediately came to mind. There are plenty of options for changing out the handlebars. I wouldn't worry too much about making everything work. If you purchase it from a good bike shop, they should be able to switch out the proper parts for you with no problem.

Mustache bars are most commonly used with bar-end shifters to begin with. Like this:


You could also buy some mounts to mount them like thumb shifters. Paul and VO make some. You could also change them out for trigger style shifters. If you decide to use a different type of brake lever, you will probably be better off changing the brakes out to the mountain version (you may want to think about upgrading to hydraulics) to give you more lever options. Some other handlebar types to take a look at would be city style bars, H-Bars, and butterfly/trekking bars. Note that there are many variations of all of these bars and different price points.

Again, I would find a bike shop that carries Salsa that you feel like you can trust and talk to them about your options. Keep in mind that depending on the type of handlebars used and how sporty of a position that she prefers, she may need a larger frame than she would for drop handlebars.