What's the real difference between a Digital press and a copier? - depends on what you call digital press as many devices that don't fit in to category of conventional offset press and not exactly copier/printer being called digital presses. Machines like IGen, Kodak Nexpress, Canon ImagePress and other machines that rated to output significant amount of pages per month are called Digital press while they are really a digital printer on steroids but share the same principal - new image being created every page you print, it could be different page every time and cost per page does not change from page 1 to page 10,000. HP Indigo also falls in to class of digital press, while it built around offset press, it uses liquid inks that packaged in to something that look like hairspray bottles other than that, per page cost does not change from page 1 to page 10,000 (it is not recommended that you print large amount of the same image as it could create a damage) A DI press also often called a digital press, DI is more or less offset press that use inks just like any offset but waterless and plates are imaged right on the press, farther on it acts like highly automated offset but unlike digital printer this is not something you print 50-100 pages on. First page printed will have significant cost as it will carry cost of 4 plates ($40), time and materials it took for makeready ( like wash-up from previous job) but as run increases, cost per page goes down significantly. There are more, just like IGen and NexPress there are InkJet presses on some serious steroids that chewing up huge rolls of paper, looks like an engineer smoked some serious stuff, looked at HP DeskJet and desided to magnify it by few hundred times, they are also considered Digital Presses. If you ask which particular Digital Press, guys here can tell you more details about differences of it VS copier. As far as the list of machines you had provided, except KM6500 (depends on package and mileage but sounds like overpriced) all of them are office copiers and it would not be wise to get them involved in printing shop. Docucolor 242 / 252 is good machine under $10K, I would recommend 242/252 rather 240/250 as there was some bugs worked out, for the same reason better take KM6501, also really important that you take machine with fiery rather than with basic built in controller, you will be glad you did.
"We've also heard good things about the xerox docucolor, but have been cautioned about getting a contract with xerox. Any thoughts? - There is nothing to be cautious about contract with xerox, as we discussing used machine, xerox just as any other will inspect your machine and would have to bring it up to their standard before giving you a service contract. "Bringing it up to the standard" will be at additional charge and with xerox it could cost twice as much VS similar job from any other manufacturer because they think they live in their own world and there is no competition Other than that they make nice equipment but KM-6501 is awesome piece of equipment and their service is highly regarded as well. Best starting point - come up with the budget, lets say $10-12k (this much should get you really nice DC242/252 or KM6501) call Xerox, call KM ask them for Service Contract types/rates, ask what is their response time, how many techs do they have in the area that trained on your particular model. Generally - the best machine will be the one with better service tech supporting it... More questions - bring them on.