I would leave Win 7 on it.
If you have a local computer service Co - they can do a fresh install using either your existing license or they usually have them spare.
OR - does it have a "return to Factory settings" function?
Whichever, it will clean out the registry which by now is probably huge. This will also speed boot-up.
Once you have the fresh install - fit some extra RAM (8, 12 or 16) and then start reinstalling software.
You could install Office 365, if you have other machines, this becomes a reasonable cost. Or - get a copy of Office 2013-2019 for a once off cost.
BUT - downsides to this:
1) The HDD might have limited lifespan left, and if it fails...................... (you could get a cheap SSD of 500gb and get the shop to fit it)
2) Your battery might not now be in the best of condition.
Me - I'd throw a little bit of cash at it, but not get carried away.