The rust on the crank only matters for cosmetics.Try removing the rust with oxalic acid,it's of course easier if you can remove the cranks but not necessary.And if you change it I would go for duraluminium cranks,Stronglight was the brand on the better Peugeots.But than you need to change the bb also.But I live in a place where old Peugeots and Motobécanes and good beer and chocolates are plenty and cheap even with a strong euro.