Designing CAN for a 3.3V system
No influence on the CAN bus if in read-only mode.


The CAN controller which is (should be) integrated in your MCU most likely has a listen-only mode. When you set the controller in this mode, it will not ACK frames.


  Terminating resistor - a unit may sometimes need a terminating resistor. Is there a method to overcome that?


The most convenient way is to put one such 120 ohm resistor on your board, in series with a jumper or similar, so that when the jumper is present, the resistor is connected between CAN Hi and Lo.


  Voltage - there are not too many 3.3 V operated CAN transceivers. Is it worthwhile to look for one to save a regulator or is the special transceiver with higher cost and performance worth it?


I don't think 3.3V transceivers are more expensive. As usual, the price depends more on which brand and which supplier you specify, rather than the details of the actual IC. Though of course, if you specify some 48 pin QFN monster, it is going to cost you... 
I'm using TI's HVD232 which is small and affordable.


  Security - the system is installed in a car. The main importance is that the system will do no harm either in receive-only mode and in trasmit mode. 


This is all on the application level. There are many things to consider. For example,  what happens if your MCU flips out in the middle of a CAN transmission, will it sink the whole bus or leave it gracefully?