It is highly unlikely that a Miroflex will function well enough for use today. The shutter curtains in those cameras are a cloth that typically now snows hundreds of pinholes, even when the cloth tapes have not yet broken. The folding/unfolding action points are also usually subject to wear as well as the chimney viewing hood. You would want to check any Miroflex in person and not run the risk of the usual Ebay disapointmet. The last one a customer brought to me was returned as "unserviceable".
That is my primary concern with the camera and I don't want to spend $400+ just not be able to use the camera, hence I'm on here hoping to find more information regarding the camera, as well as any potential technicians for it. The specific camera I saw on eBay does look promising I must admit, especially when compared to the other listings. Also, I was not aware of the wear that the folding action can have on the camera, thank you for letting me know about that detail.
The curtains can be replaced in most focal-plane SLR's from the period-- is there something about the Miroflex that makes it impossible to replace?
Presumably there shouldn't be any issues with replacing the curtains and some users have done it themselves from what I've gathered online, but I am not very mechanically inclined to replace it myself.
9x12cm is no longer very common though.
It is becoming less common, but Foma still produces it in the format for a fairly affordable price, plus there is a lot of expired Russian films in this format as well on eBay for cheap. I've bought some and they've worked decently, just needed to add some potassium bromide to mitigate the fog. At the moment, I have a good amount of 9x12cm film, but I just lack a camera I like using in this format. Nonetheless, I am considering shifting over 4x5 in the future once I find a camera I like for that format. One of my biggest faults in photography is being very picky on the camera I use lol