I’d be very cautious about buying a 280zx for someone who is a 240z fan.
Why ?
Like Sean I am a fan of both, it's not an either/or question.
Don't forget his Dad is no doubt of advanced/ing years and will not want to go tearing around at full tilt everywhere, dealing with heavy unassisted steering, feeling every lump and bump in the UK's ever worsening pot-holed excuses for roads. I know which one I would prefer to drive in older age, especially over any distance and it's not the 240Z.
Track day or drag racing maybe a different story but not relevant here although I will throw in that my standard factory spec. 280ZX turbo will pee all over any standard factory production 240/260/280Z in a drag race.
Out of interest have you driven good examples of both cars to make a valid comparison ?
Anyway, an academic question in some respects as the budget is only £10k and as already mentioned you won't get a roadworthy 240Z in budget. 280ZX or bust as far as Datsun brand is concerned on this one.