Hi Broken-Form,
The ES-9 from Expert Sleepers is still on my wish list however I also can't comment on it since I don't have it.
If you want to start easy and small then the Befaco Out v3 is nice for starters. You can use it either as input or as output but not simultaneous, the price is reasonable though.
If your budget is a slightly bit better then go for the Intellijel Audio I/O module, it has two inputs and two outputs, it has a great LED meter (as a bonus) and this is my favourite Audio Interface module for jack connections. You also can check out my review report about it on my website (read only chapter 2 (Summarise) if you don't have much time):
https://garfieldmodular.net/index.php/intellijel/intellijel-audio-i-o/
It almost can do everything you need and it's reasonably priced. Only if you wish to have a phones output then this module is not what you need. The Befaco Out v3 has a headphones output though.
If you prefer to have XLR connections then consider the Vermona - Tai4, a great and good module, semi-affordable and good sound quality. If you need more functionality then go for the more expensive ACL - Audio Interface module that one is XLR as well and has headphones output too and has a good sound quality as well. Most expensive of the four here though.
I can all four recommend them, depending on your requirements. For something simple the Befaco Out v3 is sufficient otherwise for jack connections go for the Intellijel Audio I/O module. If it must be XLR then Vermona - Tai4 is good enough unless you really need XLR + phones then take the ACL one.
Good luck with the choice of audio interface module and kind regards, Garfield.
Edit: Removed typo.
For review reports of Eurorack modules, please refer to https://garfieldmodular.net/ for PDF formatted downloads