If it's the right size for the rider, then an immaculate and functional budget bike for £50 is a bargain. Only a bargain in your case if you already happen to have a right-sized frame kicking around just waiting to be built up at no additional cost,
I try to obtain parts donor bikes for as near FOC as possible, and I never go above £10. Don't care about size, frame gender, colour etc so long as the mechanicals are useful and compatible to my other machines.
I’ve just given a neighbours Viking a tweak and a clean, these cheap Vikings (not to be mistaken for a decent 60’s/70’s Viking) are far from good. You may just about manage to recoup your £50 for parts.
Buying complete bikes for spares is way cheaper than buying spare parts piecemeal, but there is no point in paying more than necessary for a cosmetically nice frame (as opposed to a shabby one) if you have no intention of ever using it. Weigh up the condition and value to you of those parts of the bike that are actually useful as spares and make a decision on that alone.