We debated between Windows 7 Professional and Ultimate as we need remote control and virtualization to run an older app. In the end we decided to go for the ultimate version as it also gives us the encryption option.
As for the drives we have 11 computers in the household with over 20 hard drives and I have just have the best luck with the WD blacks plus it has the 5 year warranty on it. Also this machine really does very little disk use other than the virus scans and using Home Server to back it up nightly so performance is not a big issue. Most of the time is spent editing word and excel documents.
I have a couple of seagate 7200.12 and 7200.11 and I find them noisy. I have one samsung and had drive errors on it but then I reformat and run diagnostics they go away so my experience has not been good.