as a genral rule more flow rate the better
ignore all sugestions about turbulators, turbulence is not a factor as the tube is long. I should (once i am sober) be able to find some maths about how much heat transfer there is from a lenght a tubing and corelate to your design the math is hardcore and my time is limited but i will have a look.
I recommend welding fins on there to increase heat transport (more project marks lower C/w)
If you want to calculate pressure drop i recommend the hazen williams equation which should be sufficent for your uses
http://www.lmnoeng.com/HazenWilliamsDesign.htm