I am a mechanical numpty so I can only offer my opinion from my bit, Given that you only need it for the countryside and not for cities or commuting etc i would recommend a series 3, I have a 2.25 petrol 1972 SWB pick-up, and thanks to its age it is road tax free, which is a massive bonus as paying two vehicle taxes would be expensive for me. Although we've had a few problems, which is understandable for the age we have been able to get them sorted out, as others have said plenty of parts about. The fact that your dad has experience on them is of great benefit. Leaf springs certainly don't make a nice ride on a pot-holey track or uneven road but who cares if you plan on doing a lot of driving, the pick up has been most useful for moving, sacks, rubbish, hay whatever, you just throw it in and it doesn't matter how messy, muddy, wet, or smelly it is.
Mine came from a decent chap who advertised it on auto-trader