Take it from me, my Dad owns a 2003 330ci with navi, and I'm pretty sure it's the same as the M3's navi... it's definitely not the best. Why go through all the trouble for a mediocre navigation system?
^ I agree, and its the same navi software that comes in the factory toyota/lexus units. Very easy to use, in fact, it has a toyota harness right on the back of it. This means that you can plug the factory supra harness right into it, which makes your install a little simpler.
All you would have to after that to have a fully functional unit is to run the GPS antenna, a wire for reverse, a wire for e-brake, and a wire to the speed sensor.
This is by far the easiest way to get OEM quality navi, OEM look, and easy install. Believe me when I say it looks mighty badass in a supra.