I am not knowledgeable on this, just verbose, lol. When I wanted to get a stereo camera I read a lot, so it only appears that I have some knowledge.
Focus is nice, otherwise you may have to adjust how far away you hold the viewer from your eyes. I thought most let you adjust the plane of focus, or are you referring to the diopter (much like binoculars usually have one adjustable eyepiece so you can balance the eyes). Perhaps you mean something like this:
http://www.3dstereo.com/viewmaster/sldvn.html
I've not used one of those, but I imagine they would be okay to start with or if you have really young kids that may break things accidentally. I imagine it would be similar to a non-focusing View Master. If you get hooked, though, you'll eventually upgrade.
Getting a cheap, non-focusing viewer just to get started, while patiently searching for a good deal, would probably be fine. I basically monitor sites like ebay regularly for things I want to buy. Every now and then a really good deal pops up.
I have a Kodaslide viewer, and after a few years of watching, found a great price on a barely used Realist Green Button (which I've not used yet either, lol).
My Realist 45 was a gamble, after watching for over a year. The photos on ebay didn't show the condition well, and the seller was new with little feedback, so I got it for $50. When it arrived I was pleased to see it was practically new. It functioned well, and was probably only used a few times; the bottom plate still had it's luster (and I'd like to keep it that way, so haven't purchased an ever-ready case for it), and the paint on the pivot-point of the removable back had not scratched off yet. I doubt that camera had seen more than 6 rolls before coming to me.
Do the minimum (within reason) to get started, then actively and patiently watch for the better items.
Oh, another thing on Realist versus View Master... you can cut Realist slides easily for mounting. The View Master requires a special cutter, though I'm sure you can use a craft knife if you really want to frustrate yourself.