You are right they are just buffers. Some have a slight adjustment for gain but not like what we'd see on a radio.
Used, guaranteed professional ones cost $15-30. Those also have all metal cabinets which is a plus. LOL so is "free shipping"!
That's good news about the noise!
Some things come to mind, before I try them out. I already bought a 3 pro units just to experiment with. An Extron, a Kramer, and a Sgma. But the comments below are general to video DAs.
Some have a jumper for DC coupling option, so check that it's set for AC.
Terminate inputs and outputs with 75-1000 Ohms. It's usually not critical.
Video DAs have large electrolytic input and output coupling caps. Consider inserting a small capacitor like 0.01uF in series with the antenna input lead to limit the response to RF-only. 0.01uF is 76 ohms @ 210KC and only 9 Ohms at 1700KC
Some modern solid state gear might not like the audio-range signals these boxes will pass, or the low frequency transients of 3-5V from some of them when the DA is turned on and off. It's like an audio amp 'thump'.
If that is happening at the output going to the RX, it might be cured by a 0.01uF series cap, or maybe an RF transformer, or maybe a pair of reversed parallel diodes. I'm going to check with a scope and see what the ones I bought are doing in the transient department.