My Dad belonged to the local film club way back in the 60's and 70's, I remember him saying that Bolex and Eumig seemed to be the two makes of choice. I inherited his Bolex camera and projector and they were both solid metal, built to last ! (You will, of course, need to see whether you need a Standard 8mm or Super 8mm model).
In my opinion the best Regular 8mm projectors are:
1. Bolex M8
2. Bolex 18-5 (the manual threading version)
3. Keystone K-109 or K-109D
They are all very well built and reliable and will project a nice steady picture. The M8 is both a beautiful and very well built machine. I avoid auto threading machines because they can malfunction and ruin film, especially if you don't know how to take them apart and clean and lubricate them. Bell and Howell made some good projectors as well but the bulbs are very hard to find and will cost more than the projector.
As with all projectors you will want to clean the film path, I use rubbing alcohol with a cotton cloth. I like to lubricate it with 100% pure beeswax. Rub a little beeswax on then rub it off and it will run nice and smooth.
I have an M8 and it is probably the best ever regular-8 projector ever made. Be careful to lube the bearings as it is 'pre-ball bearing' construction. Little wicks need to be lubricated with oil to keep the bearings happy.
The Leitz projector is also reliable and like the Kern optics for the Paillard version the Leica optics are good. The Cinevid has relatively large toothed wheels for film transport so that the film is not too stressed. "Double 8" only.
The Leitz Cinovid is likely quite rare. At least I never have come across it, and I got some projectors.
Also to me it does not look basically different from many competing models, including the sprocket wheels.
My Cinovid has about 19mm sprocketwheel diameter, while my Bolex has about 15mm. Too cramped to get proper measurement. Each mm increased radius gives the film over 6mm extra to bend around the wheel.
No idea about rarity, but I would expect that, at the time, Germany was a large market for Leitz. Years ago when I used the Bolex and got the Leicina, I found the Leitz even better. Both had good picture stability and excellent optics.