So I have a single-frame project and want to send it to a renderfarm that does not support OpenEXR or compositing (SheepIt). Can I simply do a 1 sample pass on my computer and still get high accuracy compositing passes like Z-Buffer, Object ID and so on, to combine them later with the renderfarm result?
No, Z pass, ObjectID and alike are not dependent on Cycles render samples. Their visual output will always be the same like rendered with 1 sample - so not nice.
It's better to obtain them in Cycles with a different method - Z pass with Mist pass or Ray-depth shaders, ID-masks with random-colored shadeless materials for example.
Or get them from Blender Internal (you want to use Full Sample there).
Here is a little table what is (green) and is not (red) dependent on render samples: