determining values on used Newell coaches?

[cdat]

I hope I'm not breaking any unwritten rule, or asking what is considered a "rude" question, if I am, please forgive me.
Is there a way of determining value on used Newell coaches? We started out looking at motorhomes, and could use NADA to get a rough value of a used mh. With Newells, every coach is different and many/most of the older coaches have had updates/changes made to them. My wife and I are looking primarily at coaches that were built between 1988 and 1994. The prices of these coaches seem to run between 30k and 150k or more, many times for the same year. How doe one determine the fair market value, I'm not interested in "low balling" a seller, but at the same time don't wish to pay more then the coach is worth. Any thoughts or ideas?
Also, is there a web site that lists specifics about Newell coaches by year and model?

Thanks
John