I found a used head for about $500 for my NA 850 (and several others
in the $1000 range) and installation was another $500. Hidden costs
include a dozen new headbolts (they're expanding so the ones you have
are no longer good) and a whole whack of gaskets and seals.

If you go to the dealer the gaskets are sold individually (death by a
thousand cuts?) totalling $hundreds but FCP Groton sells a complete
kit for about $65 and the bolts are one fifth of the dealer price.
Might as well get a timing belt while you're there and a new water
pump gasket (a local water pump can usually be found cheaper than
shipping it from FCP because they're heavy). There are a few specific
tools (and pitfalls) which could turn a tough job into a nightmare so
I'd recommend handing it over to a pro (unless you are one).

Ultimately I think you need to weigh what you think you can get for it
once the work is done before embarking. Or commit to continued
ownership since the new head (if done properly) will leave you with a
car with many years left on it but with stigma that will affect
resale.

I'm happy with my rebuild overall and hope to keep driving my 850 for
a long time.

Cheers,
blurp