It might take some looking, but buying a good used mill is always the method I have used. Mind you, I'm not afraid to delve into it and fix the issues, but I do stop short of re-machining bedways, etc. Checking out that part of the machine condition is relatively easy, though.
New cnc's are like cars, in that they depreciate fairly quickly. So, it might take you a couple of years for you to get up to speed, and by then, your new machine will have depreciated significantly. Don't expect anyone to give you next to new price, when the warranty is almost up.
Having said that, the cnc mill market is kind of weird: they are worth a lot, or they are worth next to nothing. Trying to find a good cnc mill in the $20K range is next to impossible, because it will be a really beat down 'modern machine' or an overpriced dinosaur, in most people's opinion, anyway.
Knee mills are not what I would recommend for full cnc. An enclosed machine with a toolchanger is much more convenient to use. Knee mills lack (often) a large range of Z movement, unless you elevate the knee from time to time. This might be like a 6" working height range for a given position of the knee. It is possible, but selecting tooling that will fit above the job, and reach all the way to the bottom can be a bit of a struggle.
If you've got a hot product to sell, it makes sense to buy brand new (maybe $40k on up in a VMC).
If you buy used, sometimes you might be able to haggle some kind of a 50:50 warranty where the seller pays half the cost of repairs for the first 6 months or so. This might help alleviate your fears of flakey electronics. You would still have to trouble shoot and repair it on your own nickle, though, as well as crashing the machine is your own expense.
Used is nice in the sense that you don't have to worry about a paint chip here or there, or scratching the table, or dimpling it with a tool, etc. You want to buy it to use it, not worship its showroom glory.