Windows 7 is now 9 years old... I'd go with 10
Think of all the
still XP users out there and that OS is a smidge older then 9 yrs (truth be told, I hated that OS from the get go, but I digress).
To the OP:
It's going to be hard to have a computer that will have Win 7 on it. If you go with workstation class computers, you may be able to find one that has downgrade rights to Win 7 from Win 10. Your best bet is to have an install disc or ISO (or make an ISO from an install disc, some newer computers don't have optical drives anymore) to install it yourself.
While Win 7 is long in the tooth and MS is making it harder and harder for those last 2 yrs (or yr plus some months left of support), if you keep it off the WAN, it won't matter.
Truth be told, I would highly suggest keeping Win 10 off the WAN when it comes to production computers. It is essential a rolling release OS with every major version going EOL every 18 months (that's almost as often as Fedora (13 months)). Plus, MS (and I can understand why this is) has never had a very good reputation with updates in general and now they are forced. You may be able to delay them, but you'll have to take them if you want support from MS or even some software vendors as I'm sure they will start to say Win 10 from such and such update (Adobe is already doing that for programs like Premier, Audition, and After Effects with the next CC release (and this applies to their Mac versions as well), while some on here may not use those, I actually do, so that would affect me if I went the CC route).
Win 10 has been the bane of my existence, because it's what my dad has and I have to play IT for him and he
does not have the niche programs that we do. He had one update that killed his activation for his OS and for his Office products. These are MS branded products and their very own update killed the activation. That's bad. Bare in mind, this is potentially an issue with any update that you get. That's the double edge sword of updates, supposed to help fix things, but they can also break things as well. Since most people are used to running web based apps, this isn't as big of a deal, for those that still used traditionally installed apps, not quite the same thing. Glad most of my workflow is portable programs (and not using Win or Mac OS). Less of an issue, but still have things local.