Here I'll try to answer a few.
1) The shocks you should use are either Koni or bilstein Sport
2) There may be a few but I'd rather have a harsh ride than a bumpy ride. The stock springs are
Spring---------------------Front(lb/in.)-------------Rear(lb/in.)
Stock 325i-----------------------106------------------------245
Stock 325is----------------------118------------------------265
Stock 325ix----------------------106------------------------308
Stock M3-------------------------114------------------------314
Stock M3--------------------------140------------------------250
Dinan Sport-----------------------172------------------------300
H&R OE Sport---------------------180------------------------380
H&R Sport-------------------------180------------------------380
H&R Sport (29664)---------------200------------------------285
H&R Race-------------------------315------------------------ 570
H&R Coilovers***----------------340------------------------380
Eibach Pro-Kit--------------------102-------------------------277
Eibach Race----------------------160-------------------------445
IE SIII----------------------------315-------------------------570
Koni Coilovers-------------------342-------------------------407
Cosmo Coilovers----------------450-------------------------560
Tokico----------------------------165-------------------------265
TMS J-Stock**-------------------680------------------------1026
Alpina(est.)---------------------~200-----------------------~350
FK 60/60 cup kit-----------------379--------------------------379
Vogtland Standard***--------115/197-----------------------360
Vogtland CS***---------------137/200-----------------------400
Vogtland--------------------------225--------------------------360
**= Alternate rates available
***= M3 Application
(Compiled by kingmanjr00 on R3vlimited.com)
So you can adjust the spring rates from there.
3) It depends on what kind of springs you get. H&R Race are around $350. There is a group by on r3vlimited.com for springs at $205. It seams like a good deal. It ends in 5 days.
http://www.r3vlimited.com/board/showthread.php?t=116187 and they are pretty good spring rates too! Good luck.