Peter, here are some thoughts for you:
1 There are a number of folks who have had great success with installing Win 10 on older computers (5 years old or older) however there can be problems in putting "new wine in old bottles"
2 Many folks believe that Win 10 works better when installed on a new machine
3 Win 7 works well on older machines…and is supported by Microsoft with security patches up til 2020
4 Ubuntu or Mint work well on older machines as well as new ones (if you prefer Linux to Windows)
5 To check if the mouse problem is operating system/driver related, try these two steps, first use a wired mouse not a wireless one, reboot and see if there is any improvement. Secondly, download Linux Puppy, burn a bootable CD, and boot from it, if your mouse now works Ok then it's pointing towards your Windows operating system/drivers as the potential fault.
6 The economic decision on repair or replace is a common one and depends to a certain extent on the contents of your wallet!!!
Many of my customers just don't have any spare cash to spend on new equipment, and repair therefore is their only realistic option, however I have quite a few (usually younger) customers who are quite happy to consign a perfectly satisfactorily working computer to the recycling utility on the basis that their computer is 12 months old and last years colour! and they demand a new one!
7 Bearing in mind that a new desktop computer can be purchased (here in the UK) for around £150 (GBP) or so, with "laptops" running at similar prices then many folks are happy to buy new and regard their new computer as practically disposable…. The prices in the USA, I believe are considerably less expensive than over here, so this may make the decision easier for you.
8 It's my personal view that modern computers are not designed or built to last for a great many years ( or at least with longevity as their primary design objective) or to be constantly upgraded or repaired….. it's a throw away world I'm afraid!
I hope these "disconnected jottings" will help you in arriving at the correct decision for you.
Regards
paws