I repowered a 1989 22' seafarer from a carbed evinrude to a 200hp HPDI, when we turned it on we did not know it was running. It is not as quiet at idle as the four strokes on my 265 Express, but you can easily have a conversation at the helm. At cruise most DFI's and 4strokes are about the same volume, plus the sound is behind you. I have noticed older optimax's were a little louder at idle then newer versions. I believe they perform the same, just an observation from the dock and being on a few boats with them. I do agree that a DFI may suit you better, plus lighter engines and a higher cruise will be better for running around with a charter and all yours/their gear. Grog's advice is not wrong either, being that it is cooler where you fish, to extend your season, a pilot house would be a nice touch and heat is a plus. They are not necessary, but do throw options into the mix. Regardless you need to think engine, all 3, the E-Tec, HPDI, and Optimax are very good engines, a lot of us are partial to certain brands, so choosing the best for you would first be determined by what you can readily have serviced where you will use the boat, as well as what mechanic you feel comfortable with working on your boat. If you can't have it serviced locally, eliminate that brand, you do not want to be trailering a boat in search of a mechanic and lose charters.
As to the question about what year Grady's can accommodate 4strokes, most with brackets cannot with the exception of newer gulfstreams. Most boats prior to the introduction of a eurotransom will not have the displacement aft to accommodate the weight. Having said that, that does not mean the transom structure itself(transom core and stringers specifically) were built for that weight, to answer that you need to contact Grady themselves, their customer service can tell you what they recommend weight wise on particular older models, they are very helpful with this info. There is no exact year to answer your question, yes models that have been redesigned since the introduction of 4 strokes can accommodate that weight, but some models that were redesigned while bigger and newer DFI engines were out should also be able to accommodate the weight. Each year will vary according to model since they are not redesigned every year. Most Grady's should be able to handle DFI engines, they may be right at the max though so I would still check. My HPDI was right at the max Grady said my 1989 22' Seafarer could handle. I agree with their advice, I feel safe on the boat, but I have been on the same year boat with a four stroke and do not feel safe with the added weight and how the boat rides and sits in the water. Of course there numbers are suggestions, but they are not random numbers, they come up with them to provide owners information that will keep them safe on their boats.
