I used to own a 310d of the old boxy type. I'm almost certain those vans were never manufactured in 4x4. They are tough and reliable, but even the 100hp model is not fast by todays standards. 60mph is comfortable, 70mph is possible. We used to load ours at least a tonne over the maximum weight and it never complained. Mine had 350,000 miles on it when the timing chain let go. We put a new head on the engine and sold it soon after.
The 308 is bearable in terms of power, the 307 for me was only good for town driving.
The chassis is strong and well coated, I've never seen a cracked or damaged one. The cabs are made of thick metal but they do rust, particularly the bottoms of the doors, and the lower front corners where the lower grill and wings (fenders) meet.
There were a few guys in Europe putting some of the g-wagen running gear in them to make them 4x4, but I have no idea how complex a job it is.
