The page on perquin is about ISO14230 (KWP2000) slow and fast init.
In the standard ISO9141, you just send 0x33 at 5 bauds and the ECU answer at 10400 bauds. I tried the three init in my car (see old version of code) but it does not answer, even to the regular 5 baud so it's not a timing problem I think. Timing in the software serial is in fact 10416 baud, it should be close enough. It works well at 9600 baud and a terminal. Also talking OBD protocol, it's a half-duplex protocol, there is only one pin, the K one, you can not read and write at the same time. Unfortunatly I have never been able to test it IRL, just emulating it, so I don't know if it will work 100% or not
For the 5 baud init I monitored on a scope and the timing is right to the millisecond. I monitored the K line in my car with a trigger on it and it does not move at all after the end bit of the 5 baud 0x33.
The AT90CAN dev board is $50 and I would have to re-write almost everything from scracth, and the CAN protocol is more complicated than ISO so yeah the ELM327 will be a good choice I guess, but it add $ to the cost of the OBDuino. I'll talk to the ELM through the pin 0 and 1 because of speed of 38400 baud or even 115200.