I'd be very skeptical of old second-hand laptops. They'd probably have some faulty hardware somewhere - which needs replacing. So you're actually looking for refurbished laptops, unless you'd like to "fix" them yourself. For refurbished, you'd never get anything much cheaper than the cheap new laptops - otherwise they'd not have refurbed it in the first place.
There's a further problem with old laptops: Compatibility of components. Usually with PC's the components are interchangeable to a degree (e.g. RAM chips, CPU's, grapics, etc.) - though you do get some variations (especially between different eras of computing). These differences are much greater and much more likely in laptops however - thus the chances are slim that components from one laptop would fit into another. You'd need to get 2 which are very similar indeed before you might expect such interchangeability.
There's another alternative (since your requirements are so small). I've read about a new motherboard somewhere - or rather extremely micro motherboard with built-in graphics, cpu & ram. It's supposed to be used as a hobbyist "pc". I think it's manufactured (or at least designed) in the UK. Just can't remember the name in order to do a google for you. But as far as I can remember it was dirt-cheap (around R100-R200) and had something like 512MB RAM and a 1.2 GHz single core CPU, some micro USB connectors and a micro-Video connector (can't remember if it was HDMI/DVI/VGA).
I suppose your biggest problem is the screen - old LCDs are rather error riddled as that tech was quite new at the time and dead pixels would be the norm rather than the exception. And I've not seen an old CRT in years.
I do have an old Laptop which I'll PM you about if you're interested. Perhaps advertising on something like Gumtree or so BidOrBuy - though the prices I see there would put you in line of my original suggestion.
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