Thought I'd jump in here on this one and weigh in quickly since I recently completed my first shock rebuild on a 1987 CR125. I don't know if they are both KYB units, as sometimes back then Honda would spec Showa components for the 250 and 500, but KYB for the 125. Regardless, they must be similar spec.
First off... why does it need to be rebuilt? Is it leaking oil? Won't hold nitrogen? Sagging? Many parts can be ordered directly from Honda, but you likely cannot get the piston, seal or bushing. However, you may be able to get them from a KYB or Showa distributor. I didn't need those parts which weren't available through Honda, but found that Factory Connection had a bumper for me, and, later discovered that Rate Tech has LOADS of parts for these. I'd say contact Race Tech and ask them first.
Secondly, with the factory manual it was a pretty do-able job, but there are some sticking points which ended up eating a lot of time. I think you could do it yourself, but you'll definately need a manual (the factory manual, not the Clymer one which didn't have enough detail on the shock). That said, $160 for a full rebuild I think would be worth it... and as another poster mentioned, 99% of used bikes need or at least could use a shock rebuild anyways.
Lastly, those are fun bikes. If you get the CR, I think you'll enjoy it!