Streamer with built in DAC...is there much difference between brands?

Why don't you listen to your buddy's streamer and find out yourself. All the info you provided "one that cost well into the mid (1-100) thousands", "I don't remember exactly what brand he had before, but it was around $300 or so." is simply rediciulous.

I have tried streamers from $300 (I have a Bluesound Node) in relatively small increments all the way to $22k (I have a Aurender W20SE) and each increment has been worth it (finding the best at each price level). But as @soix says, it depends on the rest of your system. 

By the way, just for fun I did connect the Bluesound Node to my $17K DAC, $22K preamp and $38K amp. It sounded pretty good.... but not like my $22K streamer. 

An optimal compliment has occurred in my systems where the investment in Streamer = DAC = Preamp = Amp. Not cast in concrete... but I wouldn’t be looking for a $10K streamer with a $500 DAC... but I surely would not be looking at a $500 Streamer if you have a $10K DAC. It’s easy to dismiss the value of a streamer... so you should be looking in the same neighborhood as your DAC. 

We have come a long way since the phonograph.  It’s hard to find a bad sounding streamer/DAC these days, but if one does the homework, it becomes clear there are many variations in features, technical execution and specifics of the resulting sound.  A lot comes down to personal preference, including how much to spend.

Understood on the rest of the system but is something like a WiiM Ultra with a $500ish new/used external DAC (Topping D90?) going to be a better option than an $800ish Eversolo DMP-A6 Gen 2 using the built-in DAC?

@asahitoro my guess in that particular case like most it will come down mainly to personal preference.  Features matter as does details of the resulting sound in a specific system/setup.  Each case is unique.