Some comments:
I think that it depends upon your specific situation.
I'd say that an 8 hp wouldn't be too hard on the transom in weight or power but you may not need that much power unless you are in a high wind, waves, or strong current/tidal situation. Typically on calm day, 6hp will easily get you faster than 'hull speed'. If the currents and tides are typically faster than hull speed......hmmm.
1) The weight of the bigger engine may lower the stern and you may get gellcoat blisters IF you leave the boat in the water for the season. This has happened with my 6hp 2 stroke but on the new style, the batt, outboard and gas can are all in the stern area.
2) The issue of charging batteries depends upon how you are using the batteries.
For example, if you are worried about running lights only, then get the LED replacement lights. They are bright and they use like 1/10th of the electricity that the O/E bulbs consume. If you are using a bit sound system, GPS, VHF, HDTV...etc....then you need something else. If you keep the boat in the water in a safe location and sail on weekends, then maybe a solar panel is good. They may seem expensive but they are cheaper than buying an outboard.
3) The 2 stroke oil is no more of a major expense than the 4 stroke oil. I have used about 1/2 quart of 2 cyc. oil in like three years. The gas and boat maintenance is more expensive by far. Why, the spark plugs have cost me more.
4) This is a sailboat.