Just get a device which is supported by one of the alternative Android distributions, build a stripped-down image for the thing and install it. As Android is built on top of a somewhat idiosyncratic Linux base the environment should be recognisable to anyone used to such. You can leave out any code you don't want to have on the thing, add things you do want (e.g.
iptables configured before the network goes up, etc).
Using a "feature phone" with "minimal internet and multimedia capabilities" would give a false sense of privacy as the limited feature set does not negate the possibility of the device leaking all sorts of info to the outside. After all, "minimal internet" is still internet and as such can connect to anything out there. Given that even the most limited IoT-device can leak information to its true masters, a "feature phone" is hog heaven for someone looking to eavesdrop on unsuspecting users.
