The problem is you never know what the user is actually going to require until they start using it. So the intial fix should always be the easy one with the option of unpgrade later.
A cut down linux is relatively easy to setup up a desktop which only has a few icons on it.
Then sitting in the background you can add in functions without the user actually being aware that they are there.
As a basic premise linux/unix operate in a user/admin role with the os being protected by reduced rights.
Linux will handle pretty much any legacy hardware so a fedora installation shouldn't cause any headaches. You will also be able to get the browser working with all the normal add ons.
A wordprocessor is again easy along with a spreadsheet.
Printing again is relatively easy.
Then you see what they actually use it for. If its to slow etc you can then sell the idea that they need more hardware. If they are happy enough you just leave them. Patching etc is relatively easy and also free.
Realistically a user as described don't use 99% of what the machine is going to be capable of.