Sizing will more than likely be a crap shoot, unless you can get actual measurements from the company/seller rather than just “6L”. I have a coat that I wear a 14 in, one that’s a 12, I tried on one brand that I’d have needed a 2 or smaller, my hunt coat is I think either a 4 or a 6. Hunt coat is closest to regular American street size.
Color depends a little on your horse. Black or navy with canary points is always going to be classic and in style enough to wear. Get colors if you want/are able to spend a little more. Though it’s very easy to replace points or make them interchangeable, so you can always change those later.
I’m super flat-chested as well, I often wear a regular padded bra under my sports bra to fill out my coats, or I have a fancy sports bra from Victoria’s Secret that’s padded and push-up. (I do realize it’s ridiculous and vain but it’s easier than getting the coat taken in.) I wouldn’t go to a mens’ coat though–it’s likely to be too broad in the shoulders and the right shape in the waist.