Very good luck on the price. You may try to find a salvaged 330 engine. Aside from the engine cost, the updates to everything else related to the ECU is a killer expense too...
You say "big" like it is a "fitment" issue. The 330 enigne uses the same block as the 325, but with a longer stroke (different crankshaft, oil pan, etc, etc.....) so it will fit in the engine compartment. Changing everything else related to the engine swap will be far more expensive than just buying a new car. Except for replacing a blown engine that is not cevered by warranty (i.e., having a 323 with a blown engine with no resale value), you should realistically just buy a different BMW dude. Only my opinion. As always, modding is not an investment. Go for it.