As far as reliability goes, none of the year models will be any more reliable than the next. The dual spark ones should be more effecient, but no more reliable. The bigger alternator may benefit you, but I don't recall having any issues on my 05 when I had it. Like any bike when you are running electrics such as liners, you don't want to putt around town with everything full blast. Running down the road at Hwy speeds and either bike should run heated gear with no ill effect.
I'd pass on the 05 for $4k even with that low of mileage. In my humble opinion, you can get a newer model for $4000-$4500 with more accessories on it. Granted the bike may have more miles, but honestly I would rather buy one that someone rode as long as it was taken care of, than a really low mileage bike that is 8 years old. Personally, I wouldn't want to be into the 05 for more than $3500 tops, less is better. I always look at what I can sell the bike for, before I even buy it. You may or may not like it, or you may have to sell it for other reasons so take that into consideration.
When I am looking at bikes, I factor in the accessories that it has if any. I know I am going to add bags, engine guards, skid plate, etc... By the time I am done setting the bike up, I can easily be into it another $1k or more. $4500 will get you a nicely set up DL650 that is a few years newer.
"...When your time comes to die, do not be like those whose hearts are filled with fear of death,
So that when their time comes, they weep and pray for a little more time to live their lives over a different way.
Sing your death song, and die like a hero going home." - Tecumseh