The tank is different on the 05 as compared to the 08, it's shape is different, as well as the fact you can bolt up a factory fan kit as the radiator has a place to put in a thermostatic switch too.
The exhaust is also different, the 08 has the Akra system that from what "I" understand is pretty versatile, it can be made quiet for off roading where noise reduction is a responsibility,or the insert removed for a closed course set up. It also has a spark arrestor insert available as well.
Again, from what I understand, the 08's at least here in the states, are coming with the richer jetting as well. Which is the closed course jetting listed in the 05 manual. You'd probably go to that set up at the very least anyway.
I have not kept up on the suspension updates, but, as JohnF3 says the suspension is better. So figure that into your savings as well.
What it boils down to is this, if you have the money for the 08, or the 05. And what are you going to do with it? The 08 is the final evolution of this model and I can't think of any problems that were had on the 07's. There are a few updates on the 08 to consider, like those that were mentioned such as the new water pump seals, but, your countryman Orangeberg seems to have a permanent fix for this.
Are they going to offer you any kind of warranty on the 05, or the 08? I don't like buying used dirt bikes myself, I got burned pretty bad on the first Husaberg I bought, it was supposed to have low hours, it looked great blah, blah, blah. The thing was clapped out, and I ended up spending about $2200 USD in 1996 to go through the whole motor, even the carb slide was worn out from the guy not keeping the air cleaner cleaned. So, unless you know the person who rode that 05, and know them good, it's a crap shoot as to how badly they flogged it and took care of it.
It's only got 621 miles on it and that's very little indeed. I'd have a look at the hour meter on the built in computer. If it's got 25 hours on it, you would think it's just barely broken in. Does it still have the original tires on it? Why did the previous owner sell it?
Sorry, don't seem to be answering your question straight away, just kind of thinking out loud considering all the variables if I was in your shoes. Hope this helps.
Dale