Maybe buying used, or cheap new with windows 11 already installed would be the way to go? Computers are something you will use all the time for years, so while the initial expense is a bit tough to handle, the bang for the buck in the long run works out well. Maybe get one win 11 computer now, let wife use it, then save for another down the road a bit? Do,you need to upgrade all of them right away? Win 10 will still be usable for a number of years to come perhaps?
That depends on your definition of "usable".
Windows 10 is no longer receiving "feature" updates -- last year's 22H2 is the end of the line. So no more changes to the UI, no new capabilities, etc.
Windows 10 will get security updates until its End-of-Life in Octber 2025, at which point it will become wildly insecure to use on the internet. This is because starting with the next Windows 11 security update after Win10 EOL, the Bad Guys will know exactly how to hack your Windows 10 computer because it doesn't have the latest fixes, which will only be available for Win11.
All that aside, just as there are many folks still using Windows 7 and older releases, Windows 10 will remain "usable" until the application software vendors start making their latest versions incompatible with it.