I'm not certain what guideline IWC used at the time, but a general guideline
for a freshly serviced and adjusted watch is to keep time within -4/+6 seconds
per day. If your watch hasn't been serviced for a long time (7 years or more),
it is suggested to have it serviced.
As for straps, I like lizard for vintage watches. Whenever I picked up vintage
watches that had been neglected over the years, they frequently had a worn out
lizard strap on them. When I replaced the strap with a new padded lizard
strap, they really looked nice! Alligator/crocodile straps are nice too, and
really dress up a vintage piece like yours.
Good luck with returning the watch to its intended function (wearing it)!