One thing to remember is that Windows 7 has now exited its mainstream support phase. You'll still continue to receive critical updates (until Jan 2020), but you'll no longer receive product updates and that sort of thing. Still, a LOT of people are sticking with Windows 7. It's a great operating system, and you can always upgrade to a higher OS later. Really, with the Windows 7 OS itself, and even 8.1, there's not much to do to save on data. It's more installing an adblocker, making sure that Facebook is set to NOT play videos automatically. Making sure that if the computer comes with some type of cloud service you DON'T use it. Your aftermarket programs, of course, would be as adjustable as they are, but you are probably already familiar with them (antivirus and such). Plus, before you even go online, delete the bloatware that's sure to come preinstalled. Keep what you want, of course, but a lot of it you will probably never use.
Windows 8.1 takes a little getting used to, but it's not bad either (definitely an improvement from Windows 8). It, like Windows 7, allows you to control the updates. Windows 8.1 is also more amenable to touch screen devices, whether a laptop, desktop or all in one.
Windows 10 will update whenever an update is released, whether you like it or not. That's the only thing I don't like about it. Other than that, once the initial updates are done, which for me weren't a lot, the updates aren't really a whole lot more than the other two operating systems. Again, though, you can't control them (you can "defer" product updates in Pro and above, but not critical updates). There are also a few more vital things to set in Windows 10 to use less data in comparison to the previous two OSs.
Assuming that you want to stick with a desktop, if you aren't much of a gamer and are not concerned with upgrading your video card and other peripherals that are occasionally needed to be upgraded for gaming, those all in ones are something to look at. They are becoming much more popular. They aren't very upgradable with regard to components, but again, unless your a gamer, that's not something that would really matter. In such cases, memory would be the only thing that I might want to upgrade at a later time. I know you didn't ask for advice on the kind of desktop or anything, just the OS and how to set it up to use less data, but I just figured I would give my ideas on them, anyway. LOL.
Ah. Well, in that case, a desktop would certainly be the way to go. I have a 21.5 inch monitor. For me, anything bigger would just be too much, though I did connect it to my 40 inch HDTV once for the heck of it. LOL.
Computers on eBay are a risk, for sure. I sold one once, but have never bought one on there. Most people that sell their computers on there are good intentioned, but once in a while you'll get someone that will try to slip something by you.