I'm currently mid main quest, about to head off to Skellige.

When B&W comes out, would I be able to access that story line before I complete the main quest?

Is there a minimum requirement to starting the B&W story line (in terms of stats or completed quests)?

Blood and Wine will be self contained, as is Hearts of Stone. That means you may start it any time you wish -- it has no bearing on the main story -- but it will be tailored to characters that are around level 34.

I'm level 19. So assume that just means some of the quests I'll have to come back when levelled up enough.

All of the (B&W) quests, not some. If it's tailored to level 34, that's where the levels start. But yes, you will have to level up some more.

I don't think you'd want to try ANY of the B+W quests (or any HoS) at level 19. Sounds like you need to progress quite a bit more before starting either. You can go to Skellige and do the main and side quests until you get to the "Isle of Mists" quest line. Then see where you are as far as level goes. Once you start into the Isle of Mists questline, you pretty much have to continue on to end game, which is the point where many have started HoS and plan to start B+W.

You don't have to continue to the end of the game at all, in fact there are several high-level Witcher contracts which are mostly designed to be finished after you're done with Isle of Mists. You're thrown back into open world right after leaving the constrained locations without finishing the game.

Ok. I think I'll hold off buying the expansions until I'm leveled up enough then. May be on sale by then!

That's one thing I love love love about Witcher 3. On the quests, it tells you if you're going to be hopelessly overmatched, so that you can prioritise your quests.

I wish all RPGs would do the same.