AT90CAN with an MCP2551 transceiver. I used the AT90CAN because Atmega had a simple CAN example and i got tired of buying expensive CAN bus tools for my ECU work ( plot twist i still use the more expensive tools ;) ) I have got a ARM version, used the Luminary CPU for (yeah good choice there)

Used this at 1MBps CAN on fairly heavily utilised buses, i redid a bunch of the Atmega library so that it'd be more efficient as it was dropping far too many packets.

With this you can build a CAN sniffer, reader , J2534 , J1699 , reflasher all that stuff.

But first off, I can't release the code that does the actual J2534 firmware reflashing, there are lots of places you can find out about how to do this, so you don't need it from me, unless you're too lazy to google it.. But there is enough skeletal code to build upon this to do it.

There's basic ISO 15765 CAN stuff you can easily build upon, and there are lots of dev boards out that like the DK90 that are useable for this.