There is a large section of information on xlr8yourmac.com that specifically deals that the Liquid Cooling System leakage and user's results in getting repairs/system replacements from Apple that you should be aware of should it develop problems (I assume by reading your post that you are buying one that is not currently leaking). In some cases repairs were refused, some had all repairs covered and others had the whole systems replaced with stock brand new MacPros, but I don't remember anyone noting in their reports that original or secondary ownership was an issue. One good thing about that section of info is that they also give info about two different supplier for the pumps used in the liquid cooling systems and how to i.d. them, as one is more likely to fail that the other it seems so that might help make your decision.
It is definitely worth reading-- a few months ago I gave this link to a similar question here on MR and based on the information provided and some perseverance the OP eventually obtained a replacement dual 2.8Ghz MacPro. Can't say it would work the same should yours begin leaking obviously, but check out the section
here at xlr8yourmac and do a forum search for a large thread about liquid cooling system leaks for more info. And remember, just because we hear about some leaking there are still far more that have not and probably never will! I have a DP 2.7G G5 as a backup machine that has pretty much run 24/7 since I bought it in early 2005 with no hint of leak problem (knock on wood).
Good luck.
