It depends on which engine you are looking at
The 200 engine was uprated to 184 BHP from 163 BHP
The 350 engine was uprated to 302 BHP from 272 BHP
The 280 engine stayed at 228 BHP but became known as the 300 later on.
The facelift was fairly subtle cosmetically, with more modern front and rear bumper mouldings. AFAIK chassis and suspension remained largely unchanged.
CO2 outputs moved around causing some models, especially with larger alloys, to fall into the top road tax bracket around 2006/7.
So decide your budget and start trying a few! One thing to bear in mind is that the post 2008 350s will not be in the 'danger zone' for potential balance shaft sprocket failure.
For what it's worth, I tried a 200k (184), a 280 and a 350 (272).
The 200k felt 'wheezy' and only has a 5 speed auto box, the 280 was nice and fast enough for most, but for me the 350 was 'the one' and I felt it straight away. The AMG55 was out of my budget and running costs worried me!