Piano or Keyboard?

I’ve been interested in learning how to play the piano, but I have nowhere near enough money to afford one. I’m wondering what major differences there are between digital pianos and keyboards. By not having the full number of keys, how restricted will my playing be? What would be the minimum number of keys you would go to while still being able to play a variety of stuff?

Last, how long do you think it will be before I start to get the hang of it? The only music experience I have is playing guitar, but I don’t know how to read music at all, I either look up tabs or play by ear.