I'll agree that second hand desktops can give a lot more bang for buck, but if you're spending <£100 on a laptop:
A: It'll be of a certain vintage
B: If it's a popular one,being of a certain vintage that means *tons* of teardown tutorials on Youtube
C: Parts will generally be cheap.
Broken screens can be replaced without too much trouble, just a little patience. Same for trackpads. Batteries are cheapy cheap, and the power input? most are on a daughterboard that can be swopped in a fairly straightforward manner.
I mean, don't get me wrong, I'm a twenty year, died in the wool IT tech so I have certain skills and attitudes that make me think things are easier than they are to a layman, but on the flipside, I fucking hate working on laptops - all those fiddle bits - and even I have done literally all of the above, while drunk. if I can do it after a bottle of Merlot while singing along to Death in Vegas songs, you genuinely can too.
Buy something really cheap, and broken, and just try it. Chances are you'll spend a takeaway dinner and an evening in the pub on it.
Worst case, you have a brick on your hands, but have learned something.
Best case, you can charge beer for swapping screens out.
Powered by red wine and anger at laptop manufacturers for using plastic clips bloody well everywhere.