I agree with Buddy -- you probably wouldn't use a dolly very often. In fact, you probably wouldn't use a steadicam very often, either. Between the two of them, though, I wouldn't want to be without a dolly. Though I personally only used it a few times, on a feature film, I love every one of those shots.
You might use a shoulder-mount a little more often. But I think that's only if you're somebody accustomed to having a camera on your shoulders.
Like the others have said, a tripod is a must-have; I'm assuming you were already planning on purchasing one. Spend a couple hundred bucks; get one that's not gonna fall apart.
Focusing -- considering your most realistic scenarios, you'll only be doing any follow focusing if you get a shoulder mount. With all the other options, I'd recommend keeping as wide a depth of field as possible.
Besides a tripod, the accessory I found myself using the most often was my Josephcam. The Josephcam is real sweet -- it's just a guy named Joseph, holding the camera. It turns out -- when he keeps both hands on the camera, with his elbows locked tight by his side, he can hold a pretty steady shot, while moving around a little bit. I've seen footage from friends who use shoulder mounts, and the Joseph cam is easily just as stable -- the only disadvantage is that you aren't able to use follow focus with the Josephcam, so you're forced to keep a wide depth of field.
I'll post some pics of my stuff later -- they're up in the attic.