The E46 323i and 325i have complete different engines and management system. In short, the 2.5 in the 325i is double VANOS, with "drive-by-wire" throttle, and lot more responsive with the 14 extra Hp (even tho it has less torque, by about 6 lb-ft)
Agree with Tpl, buying a 3-series, sport package and manual tranny is a MUST. I would rather have an E30 325is with the sport suspension than a regular E46 325i. The total driving dynamic of the 3-series just isn't there without the sport package. Of course, there is always the aftermarket.
Since you are buying used, should look for one with Montana Leather interior than the ones with Leatherette seats.