If you're seriously looking to scan a decent amount of 4x5, It'd be worth it to just spend the money on a new v850. I know people don't want to spend the money, but at the end of the day you have to ask yourself how much your time is worth. You could probably already have one and be happily scanning along instead of burning time trying to find a deal. Buying used is fraught with all kinds of problems as you've been discovering and more often than not, results in lost money and lost time. I would not consider buying a used scanner unless it came with everything a new one does in the box, AND had a return that allowed you to return it if it didn't work. Anything less than that and you're risking money and time.
It's a lot like people who drive around parking lots trying to find the closest spot so they don't have to walk as far instead of grabbing the first spot they see and walking in, doing their shopping and walking out. I fairly often just grab the first spot I see, go in, grab the thing or two I need, then check out and walk back out only to see a car still circling the lot (that was circling the lot when I got there) waiting for a close spot to open up.
It comes down to how much your time is worth.
I completely agree, Adrian, and I had the v850 already in my shopping cart but I wanted to do due diligence and see what can be done. That kind of price is not something to pay lightly while I'm still developing (NPI) this workflow. There are a lot of variables in this. I adopted 4x5 after having spent some time doing classic B&W (see below) and seeing how the highlights in digital are not rendering in that beautiful, infinite series of silvery shades that I saw on film. The last time that I really used film was decades ago and so this was a difficult decision. But in the end, I had to concede that:
- Only film has the look of film and
- I wanted also the perspective of a view camera
But this is, at the moment, just speculation, and when you add everything up, it's basically like buying a new full-frame camera

And there are a million of variables, including development of film. So, I have to keep an eye on the budget. I found a v750 Pro on eBay for $419 that arrived yesterday and it works beautifully. Since I already had a license for VueScan, it was pretty much plug-and-play.
I appreciate all the suggestions in this forum, thank you.