If you are looking to get a new new disco (Nov) time, remember it does not have a ladder chassis construction. I am sure it will still be good at towing, but I would rather tow with a vehicle with a chassis rather than the new mono......... what ever it is.
manual / auto, no difference on a slip with the TD5, just put in low box, don't touch the throttle and she will pull anything. If you touch the gas, then you are introuble intil you spin that turbo, clutch issues etc. Economy, I would buy what ever you are comftable with. Try a Landcruiser.