I have the 6BTA engine on my (former) boat and I did almost all of my own servicing. The QSB is not that different, and the aftercooler looks very similar from the photos I have seen.
Freshwater flushing your engine after each use (as often as a good supply of dock water is available) will greatly help with reducing the "marine aging" (the ravages of saltwater) of your engine. It will reduce the chance of an engine overheat, and will increase the time between servicing intervals. Tony Athens's website (sbmar.com) is a great place to obtain info on all things Cummins marine (other marine diesels as well) utilizing his extensive "Tony's Tips" and the various forums.
Depending on what recent servicing the former owner has done (and how it was done), if it was my new boat, I would (at minimum) service the aftercooler right away and also check out the exhaust (see all of Tony's info on "Doomed to fail" exhaust systems). If your boat has the nice white (double jacketed) exhaust elbow, where the cooling water is injected right up near the turbo, then most likely this is one of the "doomed to fail" exhausts that Tony warns about. When that elbow corrodes through, (possibly other times as well) saltwater can make it's way into your turbo and maybe even into the exhaust manifold of the engine. Either of those things would be very bad news (and expensive).
When servicing the aftercooler, even if a "pro" does the work, ensure that it is well greased following Tony's method, before reassembly. Many mechanics do not do this (even brand new ones are not greased and need to be taken apart and greased), and leaving them operating "ungreased" even for the 4 year recommended time frame (without freshwater flushing regularly) can lead to problems including the need to replace the aftercooler. Without regular servicing (including freshwater flushing), especially the liberal use of the prescribed grease, these aftercoolers can become an environment for corrosion (dissimilar metals in an electrolyte (saltwater)), which ruins the parts. Plus the air side of the aftercooler core needs to be cleaned fairly regularly as well (frequency can depend on the CCV system, blowby, etc. for your engine) to ensure a good air supply to the engine. This can only be cleaned "off engine". All cooling components should be pressure tested before reinstalling on the engine. We don't want saltwater where it should not be!!
Going from memory, Tony recommends servicing the aftercoolers every 2-3 years if they are not freshwater flushed regularly and the interval can be increased to every 3-5 years if you do. I suggest starting out being conservative and increasing the interval if you find that it is "safe" to do so, rather than shooting for the longer interval and finding out the opposite. If freshwater flushed, the other cooling components (gear oil cooler, heat exchanger, etc.) can probably go 5 years between off engine servicing, and the anti-freeze (coolant) can be changed when this work is done. (high hours may require a shorter interval).
I cannot speak to how hard it is to change the impeller on the Sherwood pump on the QSB, but if it has a key, that will make it harder. The SMX pump I installed on my 6BTA makes impeller changes easier (as long as there is access to the front of the pump) as it comes with it's own "puller" and the impeller is "splined" so no key to worry about. Tony offers some impeller changing tips as well, like lube the pump body with WD 40 and "bump" the engine prior to removal to make it easier.
Good luck with your new boat.
