Hi Anders, My 320i now has a 2.7 under the bonnet with 325i running gear.
The 2 and 2.5 engine loom and radiator are the same.
You could just swap the complete engine over and fit a 325i ECU and airflow meter and it would work fine.
However, the 325i gearbox and diff are beefier than the 2 litre items so I fitted those. The 325i gearbox uses a different propshaft from the 2 litre so this would be needed too.
Also the 2.5 clutch is slightly different from the 2 litre although you'll probably get that with the engine.
The 2 to 2.5 swap is a million times simpler than a 1.6/1.8 to 2.5 swap as i found out recently
Cheers,
Iain T