Hello guys, just picked up my 1st M3 yesterday. It's a 2003 SMG Conv with 25,000 mile. I traded my 2003 CL500 Sport for the M3. The Cl was fun but this car is sick. It's not my 1st BMW I've had a few( 95 325conv, 92 850i, 96 740iL), but this one is going to be the most fun to drive. I have a few questions if you guys don't mine. Are there anything I should be worried about with performance mods and can I swope out the radio & put in the BMW nav system w/o any problems?

congrats!!
but yes, you should be able to retrofit navigation in there without problems...
many people have done it.
and i don't really know what you mean about being worried about performance mods...?

Welcome.
You can not just remove the radio and install the Nav head unit, you also need the Nav computer and cable harness. The cable harness for a car with Nav is different as far as I remember, so you have to replace the harness from the front to the trunk (where the Nav computer is mounted). Some people did it but it is costly, if you still want to do it I can give you the link for their posts where you can see what is involved in detail.
Did you think about the small portable navigation systems?

I have a Pioneer AVIC N1 system that I took out of my boxster, but I wanted to keep everything looking as original as possible. I also have the portable TomTom unit. Sometimes I want to through that unit out the window as I'm driving. The mods I'm most concerned about is a supercharger. I plan on doing a ecu upgrade & supercharger when my waranty is up.

Welcome to the M family.

What aftermarkey NAV unit fits/looks best? Do you need to modify stock wiring harness for an after market nav?