Well, if you're not going to overclock, I'd recommend using aluminum - especially if you're going to have your machinist friend machine you one for free. XP2400's are not hot.
Aluminum sucks corrosion-wise, but gets even worse if you mix metals. Make sure that you use a corrosion inhibitor regardless of what you choose.
Here's a useful guide to
water-cooling chemistry that includes all the additives that you might want to consider.