-18fps is standard. Pretty much all cameras can do this.
-If you're manually metering, then any camera can take any stock. The cameras have built in light meters and will auto expose, but they have limitations. Most cameras will let you manually meter.
-If you look around, and are patient, you can get almost any camera (except the Logmar or the VIC cameras) for that kind of money.
-Interchangeable lenses for super 8 cameras are overrated. A few cameras come with C mounts, and the Leicina Special comes with an M mount. There isn't a lot of pro lenses that make a small enough circle to be really usable in super 8. For example, there were only 2 lenses made for the Leicina Special. And even in C mount, most of the lenses are for 16mm cameras, and will heavily crop the lens. This can be good if you have a large selection of 16mm lenses sitting around.
The results are often better with interchangeable lens cameras, but mostly because they were the most expensive options out there. The higher end Canons/Nikons/Nizos with built in zooms often have very good performance.
-Adjustable shutter angle is common, but highly overrated. Mostly used to do fade ins/fade outs. You can only approximate what the shutter angle will be. If you're trying to recreate the first five minutes of Saving Private Ryan (with the small shutter angle)... you'll need something more accurate.
-Servo zooms on super 8 are also overrated, but common. It's not like a servo zoom on an ENG camera. Most of my cameras have them, but I never end up using them.
-You don't want autofocus. Almost no cameras have it, and I don't know of any good autofocus systems made for ANY camera (including still cameras) until the 90s.
It sounds like you want a Beaulieu 4008. C mount interchangeable lens. Frame rates from 1 to 70 fps. They did make a servo zoom: Schneider Kreuznach Optivaron f1.8 6-66 mm. Mostly go for a little more than your budget. It was a cinematographer's super 8 camera. Mirror reflex, not prism. Not the best if you want to hand-hold it, on sticks it's pretty good. Batteries are the biggest problem with the Beaulieus (weird size, dedicated battery), but there are several options for external power.
There are places that sell verified working cameras. Be prepared to pay for it though. Du-all is a place to look. But they're not cheap.:
I've actually had really good luck with buying cheap cameras off ebay. I set a low limit (say $40 not counting shipping) and bid on a few unqualified cameras. About 1/2 work as-is. 1/2 of the other cameras can be fixed with small stuff (like cleaning corroded contacts). Most of the cameras are quite old these days, so I expect them to need some work. I bought a Beaulieu 4008 ZM2 for $35 last year. Worked fine, except the grease got dirty and had gummed up the works; it badly needed to be cleaned. I got a Nikon R10 for $60, where a wire had broken in the handle (should have been an easy fix, but it's not).