Hi. I'd wait to build the drop-in and the rail until you see if you actually like snowboarding. There's a lot to learn before you try rails. Give it a go at your local mountain first. Most mountains usually have some kind of park where you can try hitting rails. If you really like snowboarding and then like rails definitely build the drop-in, it's great practice. But I'd wait to build it until you're sure you like what you're doing.
As for boards, you'll probably want a "wide" board and something sized at least at 154cm. I learned on a $50 board that I actually still have. It was basically a big piece of wood with plastic on the top but it worked! Good luck!