What frequency range are you talking about? Generally sensitivity goes down with frequency, so it makes a big difference if you need 20 mV at 10 MHz or 200 MHz. I like the HP/Agilent/Keysight 53181A/53131A/53132A series which gives 10 digits/second of resolution (12 digits/second on the 53132A, there's a thread on how to convert the 53131A to 53132A). But given your limited requirements I'm sure there will be a range of older HP counters that will do what you want.
I like the HP/Agilent/Keysight 53131A/132A/53181A series, which gives 10 digits / second of resolution (12 digits/second for the 53132A, and there's a thread on this forum on
how to convert the 53131A to 53132A. The Philips PM668x/669x series (some of which later became Pendulum CNT-8x/9x) are also quite nice. But given your limited requirements, I doubt you need to spend that much. If a GPIB computer interface, or no computer interface is okay, and you don't require advanced features like high single shot duration resolution, statistics, etc, then pretty much any old HP/Philips/Racal-Dana/etc counter would work. For good resolution of lower frequencies without long gate times you want to look for a reciprocal / universal counter. For example the PM6671/6672 can do 8 digits of resolution with a gate time of 10 seconds. It's a decent counter, but it should be much cheaper than the HP 53xxx series, in my opinion.
The advantage of picking a popular model from for example HP is that its quite plausible that someone designed a board to fit an aftermarket OCXO, like
Gerry Sweeney's OCXO option for the 53131A. I'm not familiar with the older HP counters or other brands like Racal-Dana but I'm sure someone else can jump in.
I'd suggest changing the title of the first post to something more meaningful like "Looking for used frequency counter recommendations".