I have been looking at these for some time and finally have purchased one.
Just waiting for it's arrival now.
As far as purity goes, by definition, absolute is 99.9%, because nothing is 100% For the 0.1, 0.2% this will be mostly oxygen, if it is CO2 or nitrogen.
You can spend the money on the scrubbers to try and improve the quality, but you're in a cost-benifit situation. For a 3 month shelf life I would think the nitrogen generator, if maintained and working properly, would be adequate..