It depends. Ideally yes, one would do preventative maintenance on watches on a regular basis (every 5-10 years seems to be the consensus), including when purchased without a known service history, but many people here acquire watches regularly enough that they don't tend to bother if the watch is running okay. As you note, you can consider the rarity of parts as a part of this: if it's a watch with a common movement, it's not as risky to push the envelope.
Servicing prices vary, I live in NYC and tend to pay a lot more than some other posters who have some wizened guy they've been going to for years. I would start with the presumption that it will be hundreds of dollars, depending on the watch and need for parts. Also, plan for the service to take weeks, maybe many weeks.
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