As it's a film camera ISO doesn't really matter as it's only used for exposure calculation in daylight. Zenit ET has quite heavy mirror block that shakes the camera when the shutter button is pressed. But is you plan to use relatively short focal lens this is not a big problem. Try to find as fast lens as possibe. Avoid zooms. For Zenit with its M42 lens interface you can find tons of fast and good lenses.
IMHO it's not worth to try anything except piggyback shots. It's lack of shutter speeds doesn't allow moon shots for example. It's focusing screen is too dark too. You can find a better camera with a screw mount lens for a few money. Try a practika they are far better.