u can just get springs but your shocks and struts will eventually go.
if u go with just springs the labor it takes and the cost, might as well replace your dampers.
i just lowered my 330xi with koni sports and H&R sport springs. im actually in the process of getting coilovers but i would like to sell my current set up first. the ride is good but i guess o am bitten by the mod bug so i want to go with height adjustability.
koni and bilstein carry dampers for xi and H&R for lowering springs
H&R ant KW V1 are the brands that carry coilovers
if you're intersted in my shocks,struts and springs PM me for details.
they will fit your 325xi and i've had them for 3months on my car