RS232/CAN aren't from my side a good solution.
These kind of board may not have good results in real time analysing.

For a 250Kbits/s bus, Windows is unable to perform real-time sampling. That's why you absolutely need to have an interface with RAM and controller included. This is the case for KVASER USBCAN II (the first generation wasn't able to perform more that 40% of load on the bus without loosing some frames). This USBCAN costs about 800 euros. Note that Kvaser has a range of products from ISA,PCI,USB,PCMCIA to CAN. All boards are delivered with sources for writing your software in C, DEPLHI and VisualBasic.

Now, your problem is to choose which features you would have for your CAN bus. You have to choose this according to the number of frames you have to transmit and their period. Note that a CAN bus shouldn't be loaded more that 40%. Above this threshold, you may encounter some problems on the frames period timings because of priority between frames.
To sum, I would say that if you have 10 frames to send, CAN bus is a rich solution and you may see RS485. If you have 50, 100 or above number of frames, CAN will be interresting and you could compute the needed bitrate.

Stephane