well the most common type of pump to get is the D5 or equivelant which is around $75...it's what I got, and I wouldn't do the extended tubing....now if you had an Iwaki pump , and their best one is like $175, it might be a different story.
and as for your reservoir idea...the gravity pushing down on the water that the pump is trying to push up INTO the aquarium would negate any 'gravitational advantages' from the water going down the other side.
like gaara said, you could get a peltier...put a cheap block in your line, but instead of that block cooling anything, it'll be touching the cold side of the pelt....then you'd have to have something cooling the hotside of the pelt, (big waste of effort and time) but that would essentially cool the water going through the block and the rest of your loop....but if you want something like that, you might as well just make your own cold plate, liquid cool the hot side of the peltier and get some rather cold temps that way....swiftech also makes a block with pre-made said cold plate, pelt, and water block sandwiched together in one nice package