About the BP valve, it should be set for 2400 psi if possible. Judging the longevity of a filter cartridge is tricky because of the properties of the chemicals and due to the construction of the canister. Under the right conditions of continuous operation, temperature and humidity it may well be that the life of the cartridge is as Bauer states. However, compressors are not operated in that way, not usually. A portable, like this Purus, would be operated intermittantly. During the down times, a hybrid coalescer/filter sits around subject to varying conditions. If the drain cock is left open then we should expect atmospheric air to enter the canister which will quickly degrade the chemicals. Moreover, I believe that hybrid canisters of this type harbor small amounts of water, drops or puddles that are left behind after blow down. If this theory is correct it would imply limitations to filter life through internal humidity. Bauer is sensitive to these issues and now offers a "super triplex" filter, for a price.
Bobber, I can't say that this is a "good deal". There are satisfied users of the Purus from whom we've heard on SB. My bias would be toward a model based on the Utilus 10. If you could find a Utilus for that price, in good condition, I would say "snap it up". This case is not so clear as the unit has had a fair amount of use, it has modest output and the Purus that I remember used to push a fair amount of water into the oil sump. Check if there is an inter-separator like the Utilus 10. Mine did not have one but that was ages ago.
Yes, filtration can be added down stream. Jim Shelden sells some inexpensive cans that can be packed with media and they should should do a credible job. There is always the issue of how to mount the extra stuff but this electric powered compressor suggests that a final could be bolted to the wall or whatever, and not necessarily to the frame. Just run an hp hose to the final and attach the whip to the output.
