Your HDD shouldn't overheat. If you're worried, you'll get excellent performance out of a fan on it. They are supposed to run hot, methinks.
As far as a graphics card block, if you want to spend that kind of cash, go for it. I guess you can never throw too much at a machine, though you'll not see too much of a gain over a cheaper block. The question, though, is what you are going to do about the RAM. It isn't enough to just do the chip (however hardcore) and leave the RAM alone or you'll still have a f*cked card.
As far as starting small and growing goes, that does have its merits, but I'm of the school of thought that people should go all the way from the beginning (except for pelts or other exotic stuff). That way you can build the system around what you need in the END rather than slapping it together piecemeal.
Get an Eheim 1250 or look at something more powerful. As you get into multiple blocks, you will need the head power. If you are looking for an obscenely powerful pump, look at the NurseryPro line. The NPU500 pumps an obscene amount of water and is silent. Also, you can run it out of water (inline). The only issue with it is vibration that can transmit through the case. I just put some packing foam under it and problem solved.
One piece of advice: run the two blocks parallel to each other!!! You'll get best results that way. Also, if possible, run slightly smaller line to the GPU block to ensure that the CPU block gets the most water flow (since it needs greater cooling).
Otherwise, good luck!
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