Don't be put off by the 2.0 litre too quickly - I loved my 2.0i Sport - in 'real world traffic' the 2.0 litre can be useful. The 4 cylinder engine is very smooth too and revs very easily especially in 2nd and 3rd. I found it a fun engine on the road - better fuel consumption and servicing costs. Handling wise it was excellent and it had enough performance on the road - why complicate with an extra 2 cylinders if there's enough performance for you? Didn't stop me going for a 3.0 litre though and that engine is very special in the Zed. Can't comment on the 2.2. and 2.5 but be careful as some of the 2.5 engines have different outputs - some are more powerful than others.
I've now got a 3.0si as I wanted to experience the six cylinder engine ownership and I couldn't ignore the Forum! The 3 litre is a cracker of an engine with a glorious noise though you pay for the performance in the mpg when you start to use that performance. BUT it depends what you want to do - the 3.0 litre will top 30 mpg on an easy run and more on a steady motorway trip. Driving the 3.0 litre is a different experience and is very special - however - driving the 2.0 litre is fun too - you can spend a lot more of your time chasing the red zone and the cornering ability is up there with the 3 litre. I wouldn't be too quick to dismiss it - just be careful you don't end up buying two versions one after another. . .
Try both before you make your mind up - but do consider a 3.0 litre rather than a 2.5 - there are plenty out there and with patience you'll find a lovely example. Check out the 'for sale' section on this Forum. Good luck!