Well with a build like this you don't even need a mid-tower, the board I listed is a Micro ATX, so you could compact the build fairly easily into a Micro-ATX case which can still be nice and very cheap all the while being small and portable (if need be).
A nice little option: http://www.newegg.co...N82E16811553011
As for a monitor with a $100 price tag, there are a few options I would recommend. If she is wanting beautiful picture quality with less focus on gaming. Then maybe something like this? http://www.newegg.co...N82E16824176422
It is a bit weird to look at in the pictures, but it is an IPS panel for less than $100 with good reviews, which makes it worth a swing in my opinion.
Alternatively you could go for something with a bit better response time and maybe a bit more user friendly, but will not have as much visual clarity in the picture. A few good options:
http://www.newegg.co...N82E16824009551
http://www.newegg.co...N82E16824116653
http://www.newegg.co...N82E16824236203
The monitor side of things is really a draw from the hat when it comes to a $100 screen. At that price you're going to get a monitor, not much more to get in depth about.
As for the power supply, I can't really recommend anything other than that one there. 450W might be a tiny bit excessive for a small setup like this, but it's 80+ Gold standard which means very good efficiency which is hard to come by in these small sizes. I browsed through 350W and 400W PSUs and I couldn't find anything that was cheaper and offered better efficiency. Plus, Rosewill makes good stuff!
Cheap speakers... Take a trip to Best Buy or Wal-Mart.