Let's start to build this, no? The innards of a DTV, some 3D printing, a membrane keyboard with some DIY stickers on it. Hot glue, solder iron, patience and some serious hacking can get this done. Where are the design files?
Might be hard to fit everything in, between the space consumed by the tape recorder, batteries, and expansion slots/connectors. I also seriously doubt that you could power a device like this on a reasonable number of AA batteries, but using an OLED/LCD instead of a CRT and modern lithium cells would make it more plausible.
No. Cassettes were one of the worst storage media of their age. Replace that part with a micro SD card and then maybe you're on to something.
8bit machines are good in their simplicity, but they were not perfect. Storage of the 80s in particular detracts from the experience, it doesn't enhance it.