My wife needs her system to be stable during certain points in the yearly business calendar, and an unavoidable, potentially-breaking update is a serious concern.
But why Windows 10 rather than Windows 8? Both will receive security updates for a long time, and (AFAIK) Windows 8 doesn't have the issue of unavoidable pushed updates.
The only real downside I know of to Windows 8 is its bad UI, but I'm told there are 3rd-party shell replacements which approximate the Windows 7 interface.
I can't argue either way though. I as a programmer took the plunge one afternoon around 8 months ago and never looked back. Win10 is superior to Win7 -- my girlfriend's graphical processing software (and part of her games) even started working faster after her upgrade.
All of that is anecdotal of course but strategically speaking, Win10 will be around for much longer.
Windows 10 is your best answer. It's gonna be supported very long and it's a 99.9% painless upgrade from Windows 7.
Definitely invest in a mid-to-high range PC would be my advice. I have 5-year old i7 3770 CPU and I am yet to find something that makes it choke. Only refreshment I did to my now 5-year old PC was to increase RAM from 16 to 32GB and to get GTX 980 (was 650 before). The PC is flying whatever I do -- and I'm a programmer, trust me I do a lot.