Most cameras have focus scales with marks from infinity to around 6 feet or so. If you are stopped down a bit these can be fairly accurate for focusing without using the ground glass. Framing can be a bit trickier since the wire frame viewing mechanisms are geared toward the full 9x12cm area and not the smaller 6x9cm size. Still, it can be done with practice.
As for what to pay for backs and holders, I agree with what Ian said, although I might pay a bit more for a good condition Rada roll film back. I have both a Rada and a Rollex and the Rada is much nicer by far in terms of construction and ease of use, but both will work fine if they are in decent shape.
If you are patient and willing to wait a while, good deals will turn up on Eb@y or elsewhere. Last year I picked up a set of 9x12 holders and a very nice condition Rada back in a leather case for around $20 total. Much to my surprise all of the holders had film sheaths in them, but this was not noted in the item description. Many sellers aren't sure of what they have and might not use the proper terminology in their listings so a detailed browse through the "Accessories" section of the film camera category can yield hidden gems.
There is a focus scale, so I think I'll try that and do a little bit of experimentation with the framing. I think I mentioned before - my first goal is to just be able to take pictures, higher quality will be a longer term project. Definitely exciting getting into this.
Thanks for sharing the pricing info - it is really hard to judge by what's on ebay, so this is very helpful.