Mine is a Bausch & Lomb StereoZoom 4. You Can find them in good condition for 100$. Then you need a base, and light source.
My base was probably another 100$ and the light is a cheap Led ring light, but works very well. You can get very well set up for 300$
I have forgotten most of what I learned when I researched this way back. There are a lot better brands etc, There are 100's of options.
These are older scopes that universities turn in for upgrades, guys buy them in huge lots... and put them up cheap on the bay.
I got my info back then as I mentioned, from watchmakers forums...I found out what power I needed and what to get there.
Many watchmakers use these, some use expensive dental loupes, some use a regular loupe and some... a combination of all three.
Jmb has a much better one, and I also got some info from him. It is really helpful looking at movements and things.