Hacker Keyboard will allow you to achieve a lot. All, with all CLI programs? That I do not know. There is a learning curve either way. You can also SSH out of your machine with a terminal app (recommend to look into Mosh) if you got a good internet connection such as 4G/LTE or WiFi. Then you can run e.g. Mutt remotely. Use with tmux and never close it.

As for hook up, there's BT and USB OTG.