Used Monsters....

I would like to get a used Monster. I have seen the bikes on the street and sort of became infatuated with them. What should I watch out for? Any years that were particularly poor? What size? Would a 1000cc bike be too much? I am accustomed to riding a HD Softail Deuce...probably pretty slow by your standards. I love my Harley and will never get rid of it but I would like something faster and more nimble.