He gives one of the criteria as "upgradable". I also always purchase my PCs with the intention of having them upgradable, but the problem is that I do a good enough job at choosing parts that I never feel the need to upgrade them.
So, I only "upgrade" stuff if it breaks sooner than I expected, and otherwise, I simply use the PC until it breaks and I get an entirely new one.
As a result, I always tell myself that next time, I won't worry about whether something is upgradable. But then, I choose parts to be upgradable anyways, because I can't pretend to be somebody that I'm not.
One good way to think about the budget for a PC is to break it down by expected lifetime. So, if he's spending $700 on a PC, and he uses it for 7 years, then it's $100 per year. (My PCs usually last about that long, but I'm sure I'm the exception.)