Your cause is good. But mobile phones under low cost are out there e.g. Intex, spice etc.
Also there are apps to make the user interface of android so simple, one of them I know is
Don’t go for hardware development side if you just want interface to look simple. And if you want to build low cost thing device the manufacturing, microprocessor, display, frame etc will go above 45$ at which a fine working android phone could be bought.
Move forward, build better!