The 1.8L is built much more solid than the i6's, with forged crank and heavy duty timing chains, imo this is basically done becuase the engine has to be run in the high RPM's all the time because its simply to small an engine for the car.
Either way this really has no effect on the cost of the car, i dont know to many poeple that go out looking for 4 cylinder e36's. There is a reason the e46 isnt offered in 4 cyl (in america) you know.
e30 318's are where its at, light ass car, and alot of horses can be tuned out of the 1.8L simply by advancing the timing, and chipping it.
I honestly hope you can sell your car for 4 grand, but in 2007, 4 grand seems like it will be a more likely price for 325's, if the values keep depriciating the way they are now.