Did you figure on a monitor as well? If she's older or has to sit farther back due to the wheelchair, then you will probably want at least a 24" monitor so that everything is easily viewable. That will take you over your $500 budget. Of course, if you're connecting to a large-screen TV, then ignore the previous; you just need to make sure you have a HDMI connection.
You could shave off some bucks by dropping the power supply to one of Newegg's house brand - Rosewill. They have a couple for $20 or even $30 after MIR. The SSD can be dropped down to a 120GB Samsung 840 EVO for $90. If you're only streaming Netflix and running Word, then you likely could go even lower on storage size.
You can also shave off more $ by deciding if an optical drive is needed. If she's not going to watch DVDs/Blurays on it and only stream Netflix, then dump the drive and even switch cases to a cheaper mini-ITX case. I personally like Silverstone's SUGO series, and several are available with a power supply included.