I bought the cheapest Android Phones on Amazon back in the day. Umidigi A3
But the thing was: I could not root them to make the proxies rotate.
You are better of buying a Raspberry Pi or Orange Pi, a Dongle and a Sim card.
There is a great Article about how to do that on Level 2 from August 2019.
This is what it looks like for a small setup:
But I actually buy my mobile proxies now and they work great. I believe that is more cost effective if you need less then 5-10 Mobile Proxys or want to sell them