Price is always hard to evaluate. The M500's are top shelf, so brand new would be in the $1400 range, maybe more. Being a loaner/demo unit, it got "used" for sure so only you can evaluate if you think it's worth the price. Any idea on how old it is? Dolphin/Maytronics has be updating their robot lines a lot in the last year by adding features to lower end models that were once only found in the highest end models (not unlike what happens with automobiles).
So, are you the type that's ok with buying a "previously owned" used car or do you need to have a new car?
As for the caddy, I love having mine as it makes it easy to roll my S300i around....you can certainly live without one but it's kind of like not having cup holders...when you want one, you'll be annoyed you don't have one.