Every eheim I've seen has the nozzles to stop water flow in/out of the filter included, so given those, it should be quite easy to clean/replace anything in the filter. Priming my 2213 took all of about 10 seconds. With canister filters, I tend to look at the gph and try to aim for 3x my tank size (after factoring in the head and all, theres a sticky somewhere about that).
The eheim 2217 should be capable of a 75g by itself, but I would think there is no such thing as over filtering if you add the HoB as well, with the exception of it creating too much flow for your fish (what are you planning on stocking)?