I've used a Contax 645 regularly since it first came out (1999). Medium format autofocus really works and reliably; for a subject that isn't moving or stops momentarily, like a kid, it will accurately and faster than you could manually. If the subject is moving at faster than parade speed, the autofocus won't keep up, simple as that. Additionally, most, if not all medium format cameras only have a central autofocus point, which , in terms of composition, isn't usually where you'd like the sensor placed. So they really won't do what you'd come to expect from 35mm autofocus. That said, I often find it to be helpful, but it isn't the answer to a maiden's prayers.
Luckily, my vision corrects to 20/20. I bought a microprism screen for my Contax and it's quick to focus manually for faster moving subjects and you do get a focus confirmation light, even when focusing in manual mode. You ought look at the Contax, as well. With the 80mm f2 lens, motordrive at 1.5 fps, very bright viewfinder, it's almost as good handling as 35 camera, with a real medium format size negative. The 140mm f2.8 lens is internal focusing and is a beautiful portrait lens. The Contax might be the perfect "compromise" camera.