The 325i is a truely wonderful car. I myself am an owner of one. As far as replacing the springs, I've looked at upgrading to a Bilstien/H&R sport suspension for about $650 but if you want to stick with a stock-like suspension it'll be cheaper. Parts, like all imports are more expensive than domestic, but for the most part you can get parts for very reasonable prices. The things that CAN get pricey are the little things, ie the knob that turns on the headlights, and the illuminator on the gear select (automatic tranny) A couple of my favorite suppliers include:
www.usedbmwparts.com ,
www.turnermotorsport.com ,
www.bmpdesign.com and
www.bavauto.com