Lol, you wont need a new ECU at all, thats nonsense!-All major electronic units on the car have their ID checked by the body computer on start up, anything not fitted at the factory will make the mileage readout flash to indicate a "fault". The check signal is only sent for about 4 secs after start up, so you can get round it by adding a small cheap device that delays power to the nav unit by 4 secs until after the check has been done, so no flashing odo. There are instructions on the FIAT Forum via a link on here somewhere if you do a search.
Ebay is a good place to try-they are some new ones on ebayit for 300 odd euros recently.
I brought a broken (cd player doesnt work) nav+ unit for 30 quid as a temp fix til I got a "decent" HU, but I m so impresed (and very surprised!) with the sound quality (ok only the radio as the cd is broken) that I ve decided to keep it. And I ve had some supossedly "decent" HUs-including an Alpine double DIN unit and a top of the range single DIN JVC dvd player. It evens gives virtually perfect reception of radio5 on AM, something that no other HU I ve had could do.
GPS not connected yet, so cant comment on that, but dont need GPS really as I can read a map!