I too was looking at the 2003 - 2006 models and was apprehensive due to the ABC and other issues. I have been looking at these for 5 years now. The 2007/2008 models have the second generation ABC that took the issues away for the most part and like bekcon stated they put in 4 valves instead of 3 which increased the compression ratio and horsepower. After looking for months at the 2007/2008 models I was ready to pull the trigger on one then a sweet 2009 model popped up with super low miles so I got that one instead. So after my many months worth of research I offer the following:
If you want the earlier models stay away from the 2003/2004 models as that's where most of the issues are. They made more during these years so there are more of these year models for sale and they are less expensive. Less expensive is great but look for one that had the ABC already replaced. A Carfax report is a great source of information as most of these are no longer on their original owner and all maintenance records are gone.
If price is driving you then go with the 2005/2006 models. Mercedes worked out most of the bugs from the earlier year models so these have a higher chance of less issues.
If you can afford it go with the 2007/2008 models. These are more expensive but worth it due to second generation ABC and other upgrades.
If you can afford the 2009 model years you will often find them with much lower miles and the hood, lights & airscarf was worth the price differential to me. Lower miles mostly means much cleaner and in the end I decided I would rather pay more for a cleaner lower mileage car.
Finally don't forget about the class action lawsuit against Mercedes and the balance shaft engine issues for the M272/M273 engines. I can't stress this enough! You need to get the serial number of the engine of the car you want to buy from a VIN website. You will need the VIN to enter it in this site. I used www.vehiclehistory.com
It will give you the engine model and serial number.
You are fine if your number is higher than:
M272 Engine Serial No. 2729..30 468993
M273 Engine Serial No. 2739..30 088611
If your number is below that range then that vehicle was part of the class action lawsuit. The settlement with Mercedes included replacing parts inside the engine that would normally run 6K to do once it goes out of warranty. They agreed to do this until 2016 so now they are absolved of any future issues that come up. If your seller can prove this was done and you want to take a chance if the price is right then go for it. I personally did not want to take this chance and passed on some sweet deals.
Whatever you decide the first time you get behind the wheel of one of these sexy beasts as an owner you will get goose bumps!Good luck and have fun!