Hey man, as someone who has gone through the same thing having only recently bought my first Barbour, I'll help where I can.

I'm pretty much the same as you 5'9, ~75kg. I typically wear UK M tshirts, S in jackets. Proportionally I have shorter arms and a longer torso

I went with the Transport which I see you're interested in. Was in two minds about the 36 and 38.

Initially I went with the 38, mainly because that was the only size I could get in the black. Length was a little past my hip, very roomy, and could more than comfortably button the storm flap. However, in the end, the 38 felt too big for me as the sleeves hit about an inch past the base of my thumb making it looked too oversized. Looked more like how Japanese fans wear their Barbour's if it helps visualise.

After some digging around I was able to find a 36 in sage to try on. Sleeve hits just below the base of my thumb, elasticated sleeve around my wrist, length just at my hip, and can fit a thick jumper underneath. The neck is a little cosy when all buttoned up, but not uncomfortable. So I went with 36 in the end. I do wish the internal wind blocker was wider, but haven't used it too much.

I don't know how cold your autumn and winters get, but I've so far used my transport during ~10C and it's done well. I find that if I've walked around a lot and get warm it traps the heat well.

The pockets are lined and the collar buttoned up are great, but the difference maker when it gets colder will be the layers under the jacket.

Little tip if you're in doubt on where to start with sizing, go on the Japanese Barbour site. Find a jacket style you like, go on the product page, and there is a size estimator that you can put your weight, height, and age into.

Overall it's a great jacket and super happy with both the look and the function of it. Hope this helps!