Around 2001, BMW replaced the 8-valve 1.8L engine (118hp) of the E46-chassis 318i with a 2.0L 16-valver with Valvetronic that was good for a rated 143hp. The later models are actually more appropriately called "320i" due to this fact.
This should account for the price difference...your money is getting you a much more powerful engine.
Your big clue is the headlights, the ones with the Valvetronic engine are more ovoid and less circular in shape. The older 318i headlights look quite similar to M3 units.
Wikipedia can help you identify the pre-facelift and facelifted E46 3-series...check out their Gallery. The facelifted 318is have the Valvetronic 16-valve 2.0L engine. http://en.wikipedia.org/wiki/BMW_E46
This is how a UK RHD unit looked like, but aside from the steering wheel it's pretty much identical mechanically to ours. Note the headlight housings. http://en.wikipedia.org/wiki/Image:Bmw3black.jpg