You are going to spend a lot of time with her, so make sure it fits your hands like a pair of gloves. Go to a shop before buying, so you can hold it and check if it has the right feeling.
The most important things to check are its size (related to your hands) and weight (depending on your strength and how long will you carry it) as well as its shape.
A camera should have at least two independent dials for aperture and shutter speed, well placed and big enough. Also check were is the release button and the AF-ON if you use it. The main reason why I use nikon instead of canon is my thumb.
Make sure your camera has a spot metering mode (all except some of the cheapest DSLRs have it) and you will need it if you are getting serious in photography. This is the most accurate way of metering and you’ll need it for zone system and advanced metering.
The right tool
Ok, let’s face it, SLRs are not (always) the best cameras. They are heavy and bulky, more expensive and scary.
For street and travel photography a telemetric camera is much portable and gives the same. If you are on a budget then a good compact camera or a csc camera can do the trick.
There are other types of cameras, like medium format and view cameras. They are really expensive right now, but probably prices will drop in the future.
A few more things you may want to consider:
Going full frame, a bigger sensor means bigger photosites, so you’ll get less noise and better quality in your images. The downside is you can’t use your cropped sensor lenses so it can suppose more money to upgrade.
14 bit (or 16) RAW files for better gradients and no optical low pass filter are on my list. If you are shooting sports or nature then the photos per second and camera buffer are important, as well as a huge battery to move big lenses (only cameras like the Nikon D4 or Canon 1D can do it, even if your camera is listed as compatible).
I didn’t mention price or megapixels because it is not relevant today, buy the one you can afford with more than 10 megapixels (are them still in the market?) and you can have professional results with them if you know how to use it. Remember the only reason to spend more money on them is comfort.