what's an SrA make nowadays? back when i was in, it was the first time my monthly exceeded $1k/mo.
then again, when i was in, we still had E-4E buck sergeants.
and for that kind of scratch on a 3.5" drive, you can get a Seagate enterprise hdd which should technically have a much longer time between failures (MTBF). but a basic 1TB desktop drive will run you about $50 for an OEM drive or $60-70 for a retail boxed drive (which usually has a slightly longer warranty over OEM). if you want performance, i would recommend a Western Digital Caviar Black... with 1TB of space, it'll have 32MB of cache and a 7200rpm spindle speed. if you need all out speed, you can check out their Raptor line, but i dont know if they offer 1tb or larger densities with their 10k rpm spindle speeds.
i dont have any brand preference. some people do, but their reasons are only based on previous experience that likely wont apply to anyone other than themselves and their own bad luck or poor attention to detail when they installed it. generally they're all about equal when it comes to reliability in the non-enterprise space. personally, i lean toward retail boxed, from a reliable source like newegg.com, so i'm covered in case of DOA. i just avoid refurb drives... i'll buy anything but HDDs refurbed due to poor past experience.
iirc you asked about the Seagate Momentus hybrid drive... you can use a 2.5 to 3.5 adapter kit to fit one into your imac, but i wouldnt. the overall slower average seek times on the platter portion may slow you down compared to a good performance desktop drive (due to constant head parking to prevent head crashing from a laptop's inherent portability factor). it also may not have the internal heat sensors the iMac requires, leaving your fans running full blast.
