There's no reason not to start with a BP/W now. I dove a single tank BP/W setup for over a year before I began diving doubles, and I still do some shore dives in my single tank rig. No point in buying a different type of BC, which you will then have to set aside when you go to dive in caves. All you have to do with a BP/W setup is get a different wing.
You will be more comfortable and happier if you buy a setup with a wing that's designed for single tanks. It will be more streamlined, and it will vent more easily. In the grand universe of everything you will have to buy to set up a cave diving configuration, a doubles wing will hardly be noticed. (Putting together a tech diving setup is EXPENSIVE; two high end regulators, manifolded tanks, can light, reel, high-end backup lights . . . it goes on.)
As far as where to begin, you are far from any caves, so my strong recommendation would be to take GUE's Fundamentals class. You can probably find one somewhere you can get to, or if you can find a couple of like-minded divers, you can bring an instructor in. Fundies will teach you the basic skills needed for cave diving, and show you how high the bar is on performance to dive in caves. Having taken Fundies and a wreck workshop, I was beautifully prepared for my cave classes, and I'm very glad I didn't spend the money to travel to Florida or Mexico, just to find out my skills weren't anywhere near the standard that's required. You don't necessarily have to continue your cave education with GUE, if you don't want to, but nothing you learn in Fundies will get in your way in further work. Very much to the contrary.