The point of having a 50 year old landrover now is to enjoy having a classic vehicle, and preserving a part of the nations motoring heritage.
They are a capable overlander within the limitations of a vehicle that age, the design dates from the 1940s.
My friend drove a standard Series to Cape Town and back, but that was in the late 70s, when unmolested examples were common.
The fact that you think modification of the vehicle is necessary indicates to me that you are inexperienced in Land Rover ownership and driving.
Better tyres, maybe an axle locker, and a Tirfor winch in the back and some extra fuel cans will get you anywhere in one of those.
But it is now a collectable classic, so why endanger it with a lengthy trip? Preserve it and enjoy it.