Your situation sounds very similar to mine. I bought a used road bike because I didn't know if I would like it. Then, I became addicted and upgraded everything on the bike including adding a new frame. If I would have just bought a new bike, I would have saved time and money. Now I am faced with the same decision for a mountain bike. Do I buy cheap and used or go for the what I want which is a Stumpjumper with brain.
I am also struggling over the size. Normally I would ride an XL but a guy at the bike shop, who is also the same height as me, said that a large will handle better and feel less like a boat. I can add a longer stem to make the frame fit me better. Because I have never been a mountain biker, I have no frame of reference.