.............and take it to a shop to order the key
You can get your replacement key kit from here and fit it yourself. It should take just a few seconds of your time. I've used this company before and can recommend them.
...........whether or not I should purchase this for £400-£700:
I think that nearer £400 would be the appropriate price for a fairly elderly laptop - especially if you have to pay for a Windows 10 upgrade.