I want to buy an internal hard disk and use a docking station along with it for backing up important data. The size will be around 500GB to 1TB.

I have a budget and several models fit into it. So far, they only seem to vary in size, speed and brand. These are the only things I can compare from the specs.

I guess asking for which brand is best is completely subjective so I won't do that.

I want my disk to have long life and be reliable. Doesn't matter if it is somewhat slow.

Size: Should I go for the one with highest size within my budget? Will higher density cause problems? Or should I go for a moderately sized one? Does the number of platters have an impact?

Speed: I do not want high performance. I want it to be reliable and last long. I am definitely not going to choose the expensive 10,000 rpm ones. Should I go for 5400 or 7200? Do these numbers affect longevity and reliability?

Are there any other technical and objective factors that I should look for?