Am I being too naive to think that i'm able to buy a 2nd hand Apexi ECU and just plug it in and away I go?

The ECU has been mapped for a standard version 3 STI, and thats exactly what mine is.

Or will fueling, ignition timing etc be different from car to car?

I do understand if it had been mapped for a car with mods i'd have issuese, but with both being standard is this going to be an issue? If so, what issues would I encounter?

Cheers, Mark