I might have an opportunity to buy a used Roland MP-500 for $400. I had been saving up for a new Casio Privia PX-700 (and hoping the price would drop), but this would be significantly less expensive and the MP-500 has great action. However, I'm wondering about the sound... are the samples on this older model realistic enough to justify the purchase? Or does it make more sense to go with the Privia?

I own PX700, the samples are the same as on PX110. I'd say you'd get more for your money with used MP500 (if it's in decent condition), and it looks better as well. I doubt the sound difference will be enough to warrant spending extra money on the privia.