I'd probably go ahead and use 4.3, and plan to on the project I'm about to begin. I'd rather not rely on using the legacy provider or have to port my code over later.
There may be several changes before it's released, but I don't see any of them being too drastic. As long as you write some decent tests, you should be in good hands.
If you are new to Laravel, just start with the stable version. 4.3 isn't released until (late) november and will probably change a lot before that. A lot of packages aren't fully tested/ready for L4.3 and you will probably run into errors you can't find answers for.
It is good to test L4.3 of see where it's going (and if it's going to break anything, so you can report it before being stable), but for now just stick with L4.2 and follow the upgrade guide. It probably won't take you over an hour or so.
Yeah sure, but nothing you can't do without just using Flysytem standalone ;)
But if you rely heavily on cloud-based storage you could just try 4.3, but I would only do it if you really need something from L4.3. (Or if you are an experienced Laravel user and follow the new commits etc. regularly)