Want the best, properly serviced and refurbished. Simple, you pay what it's worth. Or you compromise, and buy lesser but still good you can afford. Can't afford a Dragon or even upper end machine from Nakamichi, and want a Nakamichi, you buy a BX series machine and get that and get something which isn't too far away from it in performance. Just to name an example.
Agree. The only way you can tell it was properly refurbished is a bona fide invoice for services performed, specs achieved by a proper repair shop or individual who is known to have experience with that deck's repair. If you go to naktalk you can find some folks who do repairs. Tapeheads.net also has a lot of cassette fans with links and recommendations for cassette deck brands and repair options, site owner included.
Geoffrey, perhaps the only way to know is to have them video and audio record the deck in operation prior to purchase, and even then there's no guarantees. The only proper way is to have the deck in question Pre-purchase inspected (PPI like a used car) for performance and specs if possible. Nothing is cheap or inexpensive when it comes to maintaining these now 30-50 year old decks depending on your budget, desires, and availability.
That said, there are some less expensive alternatives - Nakamichi DR series is pretty good with a simpler later drive mechanism, then the CA, 480 series and BX series. The MR1 and MR2 are also very good depending on needs for recording.
Teac/Tascam also made some very nice stout robust decks - not nearly as sexy but workhorses. My favorites are the Tascam 122 and 122R (autoreverse I think) and the earlier 112. Almost all of the Tascam cassette decks are built really really well. The similar TEAC C1-C3 (IIRC) are similar but may be a bit more complicated as consumer decks.
The Tandberg decks are very good, TCD series and the 3034/3014, but not many can service them in the US.
As with anything nice or vintage, proper servicing and preparation can allow you to have a deck that will work for the better part of a decade. Nice things require a proper investment in time and money, how much do you want to spend??
Secondary brand offerings that are good are Akai (but who can service), Panasonic / Technics, many Pioneer (I don't like the piano key versions though), Some of the later Marantz, BIC, Dual (much less common), and maybe Onkyo.