The IQR's are very cool sleds and will always be something special. If I was to choose between the sleds listed above the decision would be easy, purchase the 2013. The 2013 is a far superior chassis and motor for the reasons listed below:
The 2005 was the first year of the chassis and had several minor issues. The steering jam nut would rip from the bulkhead if not properly reinforced. The attack angle on the track is not optimal and the front mounting location of the rear suspension needs to be lowered. The Race Department had a plate that was to be installed as a brace on the bulkhead between the a-arms. Of the 05-08 front end, the 2005 had the tallest spindles and the handle the poorest. There was no 600cc motor offered by Polaris in 2005 in the IQR, I would be interested to know what motor is in the 2005 sled.
The 2008 was the first 600cc, stock motor. The motor featured 38mm carbs and had a zerk for oiling the PTO bearing. This was a good sled that greatly benefit from a newer front end.
The 2013 IQR was a beast!!! This was the first year of the new revised ISR rules for the SnoX mod class that no longer allowed mod sleds to have material from the motors. Polaris optimized the stock motor and it put out some big HP numbers (140's). The motor features 40mm carbs and was very easy to tune. The suspension on this sled was very good and resembles that of newer sleds. Remember that this was the 8th year of the IQR and at this point the IQR had been well refined.
PS I had each of these sleds and the 2013 was by far the best.