the 323 is the same as a 325 engine wise
you need an obd1 intake manifold from a 325/328 obd2 are very restrictive
get a bigger throttle body
swap the cat and rear section from a 328 or after market i would go with supersprint decat stock 323 is single pipe were 325 328 are duel
headers no need to swap
cam wise you can use u.s spec m3 stock cam's or schrick
no need to do a ecu upgrade unless you go with aftermarket cams the ecu is self learning
without the cams you'll be around 200hp

or just m/ls1 swap
i would go lsx because your never going to be happy with the amount of money you put in to get good power out of a s50 / m52

and i really hate the gear ratio in e36 cars