B17, or any leather seat, in rain put a cover or plastic bag on it while riding. Overnight outside, always put a plastic bag or two over it. Don't let it get soaked and then ride on it, pretty simple.
and yes it will take a bit of riding to become more comfortable, but when it's dialed in and with kms in it, they are great, imo, and yes I've bought used ones too.
c17 is alright, b17 more comfortable overall though.
use padded bike shorts, they make a difference.
be very aware of slight saddle adjustments, so have an Allen key with you one b17 break in period and listen to your body. Make small adjustments and observe.