Regarding connecting a guitar, I have the Befaco INamp that I can plug a guitar into, but it doesn't really have enough gain on its own. I ended up going through a booster pedal and then into the Befaco.

As for external mixers, I got a Tascam Model 12. It can handle eurorack levels, and you can either record onto an on-board SD card or use it as a multitrack audio interface. Not exacly cheap, though.