I will only comment on your remark about insisting the dealer use a converter that buy off of eBay or anywhere else. They should refuse to do this; period. There is no quality control involved and the dealer has no idea if you’re handing them someone else’s castoff junk or not.
While I’m not saying that you would do the following it has been known to happen that a customer who is agreeable to the understanding that no warranty would exist in a case like this it’s also true that the same customer will develop amnesia and scream bloody murder if that self-acquired part turns out to be bad or fails shortly after installation.
The customer will then sue or threaten to sue at the worst or badmouth the dealer who installed that unit by claiming the dealer did not do the job correctly or something along that line.
I remember seeing a court show on TV once about a car problem and catching the gist at the start is the only reason I even watched it. A gentleman provided a set of used “big” wheels and adapters to a tire store and insisted the mismatched stuff be installed. The tire store balked, said it was unsafe, should be sent to a machine shop, etc, etc. but the customer insisted and eventually the shop caved in to the customer who “understood the problem” and did the job.
One of the hokey adapters broke just minutes later when the car hit the roadway and the car owner sued the tire shop for this. The shop lost the case by agreeing to the customer demands.
If an independent shop wants to do something like this then they can have at it but a franchised dealer should not.
That’s the main problem with a car like this; when something goes belly up the repair cost is very high. Tough call on whether or not the car is worth spending that amount of money on. Much could depend on what shape the rest of the car is in.