weird my title actually says 1987 BMW 325es
something is fishy, run the vin through www.carfax.com
being taken for a fool sucks, and its a pretty big tuning/performance difference, the motor you have is really made just for fuel economy and long life, not for modification or racing. So if that was your plan you have a good case to make the seller give your money back because you were miss informed.
the 325i motor on the other hand is gread for modifying and driving hard
the E motor has around 121hp and 172tq but with dinan chip sits around 144hp and 195tq(approximately) with a 5300-5500rpm redline
the i motor has around 165hp and 162 tq but with chip sits around 175hp 175tq (approximately) with a 6900-7000rpm redline
but the differnce is thats about all you will get power wise out of the 2.7L 325e motor without major internal work, the 2.5L 325i motor has a lot more tuning potential