Better to think in terms of a brand new screen.
Getting a £20 screen for your make and model of laptop would be hopeful in my view, every brand of laptop screen isn't £20 2nd hand.
What is the model name of the laptop?
Was the laptop thrown to the ground?
It's just you mention may'be needing a hard drive too, if all you have is a virus there is no need for a new hard drive, if it's been dropped or thrown then i can see your point.
Laptops can cost anything from £200 to £2000 so it depends on the model as to what to do.
A new screen may be £60 upwards, they've became cheaper in the last 3 years.
New laptop hard drive £30 upwards.
If the laptop cost £300 or under some people may say just buy new, if it cost £500 to £1000 then it's obviously worth fixing.
Finally, what is your diy knowledge like when it comes to pcs?
Screens are a reasonable job to replace, not the very easiest.
Fitting a new hard drive is the simpler of the 2 jobs.
Fortunately there are a huge amount of laptops manuals available freely on the net so we could see how it's done.