Working outside in, I’d suggest you consider the following:
1) As mentioned above, a potable audio recorder (Zoom, Tascam etc) is your best bet for recording. You can also make field recordings with it and bring them back to your system.
2) There are great output modules (I have a Befaco, WMD, Joranalogue is great) and they usually have a headphone out as well. You can have a separate input module if you will be importing sounds or a combined I/O module (eg. Intellijel I/O).
3) You will have to look at a mixing solution that feeds into your output module. Befaco STMix is a nice 4+1 stereo mixer that will duplicate mono L to R, with level control, DC-coupled to mix CV but no pan control. If you eventually upgrade your mixing solution, you can still use it for sub-mixes. Next level up would be a larger mixer with pan control and Aux Send/Return (eg SSF Vortices). The top end (expensive) solution would be a Befaco Hexmix + Expanders or a WMD Performance Mixer (I have one, it is f***ing awesome and transformative).
4) You could mix outside of eurorack as mixers can be quite inexpensive, but then you would have to bring all of your eurorack level feeds down to line level which is not practical. Sorry for the length of the reply. Good luck