One warning: Unity, with the need to get the mouse into a very small strip down the left side of the screen to get to menus, is not at all "Parkinson's friendly." After several months of struggling with it, my older sister let me install Xfce on her Ubuntu desktop, and she's found it much easier. And, when she got a laptop, we started off with Xubuntu because we knew that was what she wanted. You may not find Xfce as comfortable as we do, (I use it on Fedora.) but if you're setting up a computer for somebody who has difficulties with small, precise hand motions, pick a DE that doesn't require them, and you'll all be better off.