I found the EN-6704V. I think I can answer this fully.
The cpu is probably a SoC with a few arm-7 cores. The GPU is probably a Mali - again with a few cores. I would recommend NO LINUX OS except what is on it. The reason is simple. Whatever kernel is there will have several proprietary modules built in and when you swap OS, you lose these.
The EN-6704V things are available for $30 in alibaba.com. There are a number of Chinese manufacturers of SoC devices for the consumer market. Here's a few of them compared in tablets.
I have a tablet with an RK3188 - nice tablet, if slow GPU. I ran the software down for that. There are three or four proprietary modules in the OS (Android). I counted three for the rk3188 and at least one for the Mali GPU. The IP for those parts is cheap, allowing SoC manufacture. Rockchip supply Android-4.2.2 with no updates. Whoever built mine bought the Rockchip rk30 board, a screen, a few mouldings, One guy got CM installed on something similar by cp-ing the modules and hexediting the version magic. Not for the faint of heart!
The question to ask when you consider buying one of these is: "Where do I download software updates from?" and check the website.