I need a unstoppable car!

Hi all! I’m looking to purchase a back up vehicle that in the event of my main vehicle falling fowl of breaking down due to a bad sensor or other issues, I would be able to hop into my second vehicle to get parts etc, considering I live over an hour away from any parts shops. So here is my query…

Is there a diesel car, available cheaply (so not classic at a silly price) that could run with out a need of the ecu? I used to have a Citroen Xantia with the old style mechanical diesel pump that once started would run even with the battery disconnected. Is there any others out there like this? I’m a correct in saying that the diesel Peugeot 306 could do the same thing? It needs to be MOT worthy, but my main point is, if I had to use my secondary car, I need it to be drivable even in the event of it having some “sensor issues” I don’t mind a light coming on but I don’t want any limp mode or the ecu shutting down the engine! Any suggestions??