how to: modify Diecast cars???

how do u modify diecast models, where do u find the parts????
List of Cars I have and are interested in modifying:
1-18 1999 Mercedes-Benz S500, Maisto
1-18 2002 BMW 745i, Kyosho
1-18 2003 Mercedes-Benz SL500 Release Edition, Maisto
1-24 1998 BMW 740i Sport. Paul's Model Art(Minichamps)

Depends on what you want to do. With Maistos, wheel swaps are easy and you can either cut the springs or remove them to make it lower.

well my first project i would like 2 change the S500 into a Lorinser, buy the fenders and underskirts, but where do i buy these little parts??

You might want to use some body filler(or cetian types of clay) or make the parts out of resin plastic.

You can buy parts here http://www.tuning18.com/ other then that you have to make everything.