@Alissa Evans, welcome to BP!
I think the idea of purchasing a home for your parents, and as an investment, is an excellent idea. In fact, I'm on the same track! I'm looking for something for my mom--similarly she doesn't want/need much and a clean, safe place is more than enough for her.
Back to your question, consider the difference in property taxes for homes inside Bexar county and out. A key factor towards cash-flow and affordability. A number of my co-workers live in the Shertz, Cibolo, and Selma for that reason--which you honed in on.
Lots of growth and revitalization in that area as well. Not that you're focused on appreciation, but I'm sure you'll see some over your hold period.
Also, the neighborhood between Toepperwein Rd and Live Oak's Main City Park seems to be a good spot to look. Decent homes with inventory old enough that they'll need some updating. The City Park is very nice and the area has good access to shopping, hospitals, the AFB, and the new Ikea.
Best of luck and please keep us posted on your search!
-Andrew