No, your idea is perfectly rational. Somebody I know consulted me on what kind of computers to buy for their new small business that would only be used for browsing, email, word processing. I found them a store that sold used Dell and HP workstations. They got 3 Dell machines (CPU + Monitor + Keyboard + Mouse), all Intel Core i5 with 16 GB RAM, 1 TB HDD with Windows 7 Pro, for $75 / each. We spent an additional $25 to purchase a cheap 128 GB SSD and installed Linux (LMDE), Firefox, LibreOffice and GNU Cash on it. (Preserved Windows Dual boot option, just in case they needed Windows for something). This was 2+ years ago and the owner was so happy that I reduced his IT hardware budget by a quarter. I recently purchased a used HP 25" monitor from Craigslist, for $60, in excellent condition and still having a year warranty on it, whose retail price was around $500 on launch. There is so much e-waste being produced ...

It's the same as with cars - companies want brand new because then you get a full warranty and theoretically you don't need to worry about it. So that $400 you saved would be spent in IT support for your old failing hardware.

But the thing is.....old PCs are really not that unreliable. If they survived the last 5-10 years then they are probably still chugging along just fine and for a small business there is literally nothing wrong with using them.