I think you've convinced me that 35 is way to go-I've noticed Barenia bags do sell for a premiumHave a look through our leather reference library.
Gulliver/Swift is a good bet for you because you like that original (Jane) Birkin undulating look (me too). Never had a problem with rain with any of my Swift bags BUT I don't intentionally go out in a storm wearing one either.
Original Barenia may good too because it doesn't care about rain, hard to find as many have been talking-up this leather lately (then storing them in their wardrobes and reselling).
Warning though. A B 40 is most leathers is heavy to carry on a daily basis (and I have carried a Travel Bolide on a daily basis). Personally, I would try it out before you commit (unless you are fabulously strong - in which case you have my admiration).
re rain; though I doubt I'd take her out if heavy rain was forecast all day I dont want to feel her use is restricted to dry days.
I own a Fijord leather Jypsiere in gold-I opted for the fjord because of its durability however due to the colour of the bag I doubt I would chose to wear her if rain was forecast!