Here's my 1/2 nickels worth! First of all you have to go look at it. Plain and simple!! Make sure everything works from what you can tell. Try it out. Just like buying a car (new or used). If you are still serious about it, have a pre-purchase inspection done. Use someone you feel comfortable with. Since this individual is suppose to have full records, you can call that company and have them do it. If the paperwork shows it was maintained by a non-dealer, locate a dealer and have them do it. This would be if there were any updates to the equipment. Kind of like those notices you get about possible problems with your car. Generally only the dealers have access to this information. This doesn't mean that aftermarket service was bad or wrong, but that they might not have the most up-to-date information.
IMO about 10% of the asking price seems reasonalbe to pay for this inspection.
For any major purchase on a used item, you should try and get a pre-purchase inspection. We do it for houses now, why not spas?
If anybody is wondering why I recommend this, I use to service copiers and you would be surprised how many people just buy it and then get P.O on the money required to get it fully operational. Then again you may not. LOL
r100rs (rider) ps - remember a real BMW has 2 wheels