seeking used streamer/dac combo? suggestions

All of the ones you listed have their merit. The Simaudio is the oldest but still a great device. It is the only one that was primarily designed as a DAC, with the streaming card being an add on that performs pretty well. Aurender is primarily a streaming brand who dabbles in analog outputs here and there. I think they do a surprisingly good job at it. Lumin is probably in the middle with a focus on transports but pretty deep experience on the DAC side as well. 

If it were me I think I would go with the Simaudio. I just like their house sound. The others are nice as well though.

I have a Lumin P1 and like it- most of the time- there seems to be some ongoing instability with it and Roon though. Even a fully hardwired system- my laptop to the Roon Nucleus to the Lumin - meaning they're all on the same hard wired via ethernet system- has stalling, hang ups and frustrating other random glitches. Quboz/Tidal directly to the Lumin are ok. 

tidal and quobuz

@tubeguy80 i am having similar questions, nice to hear you recommend the moon 390. I would also need to use the phono stage, which i am told is somewhere between their 110 and 310 in performance, did you use it?