I can't comment much on the hardware. I only have the OS and some programs on my SSD, all the metadata and artwork are on mechanical drives with the movie (allows me to use either mediabrowser or xbmc). It seems to work fine.
Is your sister competent with this kind of stuff (HTPC, HT stuff)? My friend wanted me to setup xbmc for him and I told him I'd help him do it while teaching him how but I wouldn't do it for him. This isn't the same as hooking up a receiver or calibrating a tv and then leaving. I constantly have to fix or tweak stuff on my system....I'd guess that most of us have to tweak the system as updates come out. If the person getting the HTPC isn't familiar with or interested in learning this stuff they should stay away from it and just use something like a WD live tv or roku. HTPC is not simple plug and play.