We are going to do a dual zone mini split. One zone for the master bed//bath and the other for the new living room. The existing will be heated and cooled by the exisitng heat pump that does a pretty good job. Yes, we are going to insulate the addition well, and possibly update some of the insulation in the existing where possible without ripping down drywall unnecessarily. Our electric bill is right around $90-120 summer and winter, and we are 100% electric appliances. We will be going to gas stove and gas on-demand water heater.
OK, enjoy reading a lot of the posts on here. Try to keep my mouth shut when I don't know stuff. But, I do know about Mitsubushi Mini Splits. I too, live in the southeast in an 1840s brick house. Which means solid brick walls with its attendant advantages and disadvantages. I have an old (well, now) boiler with wonderful, quiet, baseboard heat...with six zones. For AC and basic heat, I chose Mitsubushi "Hyper Heat" mini splits to basically zone six large rooms (the gas boiler becomes the back-up heat).
Frankly, being older, from an age when heat pumps for heat sucked, these new inverter (read computer driven) mini split heat pumps are, well, great. Very efficient and allow you to "zone" multiple rooms.
Perfect, no. But I wouldn't go back. I've replaced a compressor (because of a leak), and had a couple of small issues, but these things are great. I know you are going "dual zone" but I think you will be pleased. Mitsubushi and Fujitsu are brands that seem professional.
Frankly, I've gotten so good at working with the HVAC guys, running disconnects and watching, I'm going to install a Pioneer unit for a farm worker as a bonus. Well, if I can do auto AC, this don't look too hard (the units are precharged for a certain length). High pressure gauges and vacuum pump, and you're off and running.
My only bottom line advice is get the most efficient heating and cooling unit you can get in the brand you choose.
Best of luck. (And Bosch dishwashers don't seem that expensive - the other stuff...uh, not so much.)