hey BMWboy, welcome aboard. What kind of mods do you want to do? If you are looking for.....
all around fun:
tires are the best mod you can have
power:
chip (see: $400 with a 3bhp gain
![Disapprove [V] [V]](/images/smilies/icon_smile_disapprove.gif)
)
exhaust (see: $900+)
intake (rig one up yourself)
handling:
tires
H&R Springs and Bilstein shocks (bread and butter for bmw, $700ish?)
sway bars ($200 a piece i think)
braking:
again tires are the best mod here...
slotted or drilled rotors (big price range, but they don't do a ton)
you could upgrade to e34 m5 brakes or somethign like that
i don't know, this is just a stab at your question late at night, so i'm sure i left something out important.