I have stuffed around with a couple of second hand ECU's and would recommend buying new unless you get a good clean well labelled one etc.

You would think it would be easy cause you can get the schematics off the net but it always ends up being more difficult than you think.

I put a second hand Motec M4 on my old FJ20 and could not get the computer to interface with a laptop. That sort of thing you can't check when you buy them.