If its a shitbox to use as a van, and a great "van" for tip runs, the Zaffy A seems indestructible.... ....and by god I've tried. It has an ultra annoying habit of just always working.
The Zaf B is marginally more roomy inside, but having never owned one, can't comment on reliability. Other than the well documented busting into flames of the non Elite variant due to the heater fan resistors.
The Zaf-A DTi has the well hated VP44 fuel pump that was all the rage in the mid 1990s, with its well known issues which effectively write the car off.
The DTi is also woefully slow, though I'm fairly convinced mine is knackered, and only giving me about 2 bhp. It will touch a ton, but getting there takes some effort. Its also a bastid to start when cold, but I think its down to 1 working glow plug. I think its the shittiest car I've ever owned....
....but I've kept it for over 7 years, its one of the very few that has never left me stranded, and because it is so glacially slow its a hoot to drive because you daren't slow down
Its never a car you would desire, but can't knock its versatility.
I kinda agree with the dodgy Derby builder - if its a short term requirement, get the cheapest you can that runs and has a long MOT.