I've been looking to learn piano for a while. I'd like to get a cheap keyboard that I can hook up to my 13" MBP(I've got some pricey headphones I may as well put to good use). Would it be best to get one with 88 keys like an acoustic piano? I've heard you don't need that many but I'm not totally sure why. Won't allot of music have notes that I won't be able to play? Somewhere around $100? I don't want to put down too much dough on something I'm not sure if i'll continue.