I've mounted several 7" tablets around the wall... all are Acers that I purchased as refurbs from Amazon. a couple have died after 2 years, but at $70 I'm not too unhappy.
I found some store bought picture frames that I routered out a bit for a bit.... and it looks really nice.
If I were to do it over, i'd probably go with 8"... I have a few of those now on desktops, and the extra screen size does help. I think it depends on how good your eyesight is :-)
I'd also buy a named refurb over a cheap knockoff... just seem to be higher quality. also, there is a noticeable difference between 1GB and 2GB ram in terms of performance... Not a big difference in higher resolutions. Again, may depend on what other apps you want to run.