I am a freelance developer. I have a PC, a laptop and an old testing and file server machine. I might add one or two in future.

I want to have an on-site backup machine that can handle backups of ALL these machines - file backups, MySQL backups, backup of subversion repository, etc..

When building the machine, which components should I invest more in?

Examples:

The cabinet should have lots of room for expansion.
Hard disk size should be large.
But I guess hard disk speed need not be high (?)

But other components like, RAM, PSU, Processor, Network card, Cooling, etc.. how much relative importance do these have in a backup machine? Which of these components should be high-end or large, and which ones need not be?

Some Idea of the load: There will TBs of data. File backups and subversion repository backups will at least be done daily. MySQL backups done weekly. assume 3 machines at the moment and somewhere around 10 machines in the future.