If you are pursuing open source simply because its free of charge then you’re pretty much stuck with Blender. If the need is to just make simple 2D textures the old-fashioned way, then theres Gimp.
If you are willing to spend a little money, then I highly recommend 3D Coat. Its got an affordable educational license which can be upgraded to professional at a later date and its a champion in texture painting, retopology and UV mapping. Its sculpting capabilities are not too far off ZBrush which sweetens the deal…
What about BPainter, the Blender add-on? Yes, it’s not free but, does it work in the way that you would want (meaning the zoom level thing you mentioned)? If it does, I’d go with that. This way you get to use just Blender without having to switch apps. Plus it’s a great deal cheaper.
Never used the painting feature of Sculptris myself but it would be my first choice if it works as expected. I don’t have an issue with switching apps. In fact, I’m gonna reinstall it to test it out.