The recommendation regarding the use of a good reset controller needs underlining.

We did a simple power supply with a LM317 to get 5V from a 12V battery on a remote controlled tractor. All fine till the engine cranked - the voltage dipped to about 6Volts and the processor flash went for a spin. ( It was a 89c51RD2 )

Once bitten twice shy... Now we never design such boards without the MAX1232 or the like.

The other is the ground isolation - in all these vehicles the -ve is tied to the chassis and when you use a regulator like 317 the ground of MCU is the same as the vehicle chassis. If this is going to cause a headache then a DC-DC convertor is the only way out ( I think )

Raghu