Hi there,
That's a great question and I think it may be confounding a lot of other NPOs also. I think before you spend a single dime, you should ask, what problem are you trying to solve, and is replacing your old servers the best way to go. 4-5 years is not uncommon, but it really depends on what functions are you trying to address. Your IT company may be recommending that solution, because it's probably easier to configure them all at the same time, but "saving money" is also relative. What kind of servers and apps are you running?