If it is mostly the travelling, I can't help thinking buying an expendible old diesel car or van might be a better solution than a bike.

Depends on where you're traveling. (And sometimes diesel is not an option. E.g. in Brazil, there are - or used to be, at least - no private cars with diesel engines, only trucks and buses. Something to do with promoting local biofuel use.)

I was on a Honda XR150 in the mountains of northern Vietnam over Christmas, and what struck me was that passenger cars - even pickups/SUVs - are an ostentatious luxury there. The quality of the roads and the traffic inside the cities is such that travel by car is really slow. Scooters are by far the faster, more convenient way to get around (and anything too big to go on a scooter, goes onto a five-ton truck). In a place like that, even if you could buy a cheap old car, your travel would be a lot slower and more infuriating in a car than on a bike.

But in other places, for sure.

EDIT: It's also worth considering one's ability to do long distances on a bike or in a car. Personally for me, I know that on a long-distance drive, I am likely to get bored and uncomfortable, even with audiobooks etc. Yes I have a heater, air conditioner, all that stuff, but I still get out of the car stiff and cranky. On a bike, you would think it is a much more physically demanding activity, and you're likely to be exposed to bad weather, but even after a cold or wet or too hot ride over an entire day, I somehow feel a lot more fresh and ready for action. Maybe it's the constant mental activity and having to pay attention, maybe it's different for other people, but I know that given otherwise equivalent circumstances - nice weather (or proper gear), decent road quality - I always much prefer doing long distances on a bike than behind the wheel of a car.