Parts are often available from aftermarket supply houses.

I am not sure about "Many of the high-end units don't have a self-cleaning feature". I see that high-end brands do give customer choices by offering models with and without self-cleaning. But it is not like they do not offer self-cleaning because the function damages the oven.

Some companies discontinue parts after a certain amount of time has passed, though sometimes parts houses and large repair firms have some in their stocks.

All the high end brands that I know of have self cleaning. Gaggenau, the ne plus ultra of high end, has 905° F pyrolosis and doesn't have a problem with their enamel. They don't make a 27" oven anymore, however. I think it's more the middle brands that have trouble keeping their prices down and still make a well functioning, high heat self clean. Some have self clean at 550°, which is supposed to work fine, but may need a little more work on the final hand cleaning.

I wouldn't believe CR for ovens. Their standards are about what one can do in a laboratory, not how nicely an oven cooks. Some of the older GE's were well liked while others were awful. The current ones get mixed reviews. GE is an example of mid-level.

Your biggest issue is going to be making a choice from the limited options available in 27". The new standard is 30", and the 27" ovens are made for retrofitting.

So, to your question about used ovens: As with anything else, it's how much are you going to pay for it. Used microwaves aren't a good idea because the magnetron degrades with usage. I wouldn't pay more than $500 for a pristine, hardly used conventional oven that was more than two years old, no matter what the brand (mid to high end). If you're talking ten years old, I wouldn't pay more than $200. That's basically "renting" it and putting off the decision to buy a new oven, which could pay off and give you many years of happy use.

If you do go with used, do yourself a favor and get an experienced and qualified technician to install it and check it out, to make sure it's really running right after being moved and all, and factor the price of that into your budget.

Having a used one checked out first is a great idea. Considering that ovens are fairly simple devices, it seems like a good oven should last forever if you don't run out of parts. Heating element, variable resistor, thermostat - the rest is design. I like the look of the older Viking ovens - but not fond of the new ones that look like all the others - and not crazy about the idea of putting, what is essentially a computer, in an oven - that sounds to me like certain failure in short order. Viking also provides detailed service manuals, which I found to be refreshing.

ROTFLOL!!!!!!!! "ovens are fairly simple devices" On what planet? Perhaps vintage ones were. Simple that is. Most of them were inferior in performance to the new ones, but the people were better cooks and worked around the difficulties rather than swearing at them. Ovens are the most problematic part of any kitchen, with a zillion potential issues. The parts may be simple, but the function isn't.

I wouldn't touch even an old Viking, which was potentially adequate.

If you want to go vintage and can find a Chambers in good operating condition, that might be a good option. Otherwise, your first idea to get a Jenn-Air of the type you liked is your best bet. If you're going to buy into the trouble of an oven you don't already know, you're better off with a new one at least.

Or...wait a minute. If you just need the oven to reheat take out, and you're into it more for the tinkering potential, then perhaps this would be a fun project for you. I was laughing from a cooking perspective, but that might not be of primary importance to you.