I think 5k is pretty close to what that 5648 CB unit cost new but I don’t think they make that model anymore. New stuff (I think) is all new EPA and multi state compliant. This could be why the asking price for the old school stuff is so high. Also, it might have sat empty for years. Do you have some history on the boiler?
A 12 year old CB that has been used for 12 years, even properly maintained, will be pretty tired. The fire box will probably be like Swiss cheese. Good news is you can send them back to CB and have them rebuilt. If you are in a state that does not have any outdoor boiler regulations or you are in a state that is attempting to put regulations on them and you want to get one grandfathered, it might be something to consider if you want the old school. Otherwise, I would go new. Good luck with your choice.