Buy either of WD 2 TB Green or Seagate 2 TB 5900 RPM HDD, both are almost the same , you can get the WD since it gives 2 Years warranty , but i just bought a Seagate , did not think about it so much.
I too download a LOT of Stuff , + i upload almost all the time at 10MB/s in my local network , the HDDs are running quite well . However i feel you should limit the number of simultaneous reads the HDD is having to make , that is , don't seed too many torrents at once , the read head has to move repeatedly then and it wears down the HDD faster. I seed a few torrents at a time , and have only 2 slots open in DC++.