I'd pick out ten. Prioritize, starting with the ones you want to practice on, if you make mistakes. I'd put
the rest away, so you won't get discouraged when you discover this is real work. Then when you fix three
or four up nicely, you can go back and pick out another ten. If you can, you might want to entrust one or
two of the really good ones to a pipe repairman to restore, to further encourage you, and you might get
some free advice along the way. You don't want to weigh yourself down and abandon 70 pipes.