With 4 years use and with it being stored outside as well, leading to the tarnishing, then the older model is not such a good buy now. Dog odor will wash out, but as it has been used for 4 years on a regular basis, then I would probably go for the more expensive but brand new model. As I said before, my decision would always be based on how much use the older model had had, and where it had been stored. Storing a vac outdoors or even in an outhouse leads to it getting damp, and this is not good for a Kirby.
Go for the Brand new Sentria 2 - its a good price and in the UK we would be looking at paying over £1000 for one.