Your missing the tools to install it properly and perhaps legally.

As much as I like my mini split heat pumps, nat gas is going to be a way less expensive way to fuel heat than heat pumps in this day so a furnace or boiler and air conditioner might be your best friend. (I live in a predominantly cooling environment and installed mini-split heat pumps when gas was more expensive.) If you are going with steam, or some flavor of hydronic for heat, then I'd go mini-splits. An open floor plan is more conducive to mini splits because the more closed-off rooms that you have, the more equipment you have to install. You are way ahead of most people in making the centralized vs. mini split decision since you have direct experience with the latter. If you go with ducts, be sure to put them inside the house envelope, not in a vented attic. If you are going to go with a very efficiently insulated and sealed envelope, the efficiency of the heating and cooling plant don't matter as much. As hot and humid as it seems down on your costal plain, heating is much more the issue than cooling. You might get enough cooling from relatively few mini splits since heat and humidity will move from room to room much faster than in and out of the house if your envelope is good. You might be interested in this recent mini split analysis: Long-Term Monitoring of Mini-Split Ductless Heat Pumps in the Northeast Building America Report - 1407 December 2014 Kohta Ueno and Honorata Loomis

I have no experience with heat pumps in an area that cold. I understand that they are getting better all the time, but I don't know how they will do in your coldest weather. Cost will depend a lot on how many rooms you have. If it is an open floor plan, cost will be lower. If you have lots of walls and doors, the cost will go up as the number of units increase. Ducts are bad when they are outside of the house envelope. Design your house with indoor ducts if you go that way. Ducts are bad when returns are inadequate causing pressure differentials between rooms. This, just like leaks in outdoor ducts makes an air handler into a power vent. Mini splits are inherently zoned, but what does that get you? Zoning is not going to save you that much energy with a well-insulated house. Heat transfer between rooms will be much faster than from inside to outside. Zoning for comfort and control might be worth it. I have seven mini splits inside on three systems for nearly 5 years. I live in a hot, humid environment in a home that was retrofitted with central and had ducts in the attic and crawl space. The original central system was done right with a dropped ceiling (9 to 8 foot) in the central hallway. Later is was messed up with outdoor ducts added. The mini splits perform beautifully for humidity control. Maintenance is higher with mini splits. At minimum, you need to clean filters for each unit instead of one, two or three. I am finding that maintenance of my Mitsubishi units is greater than that. Lots of dust gets stuck in the "coils" despite regular cleaning of the filters. The back side of the coils is difficult to reach if the units are near the ceiling. In addition to that, the blower cages get dirty as well. I am trying to figure out how to get the cages out to clean them without largely dissembling the whole thing. There are some characteristics of mini splits that some to not think of. Their performance measures are hard to compare to single speed central systems. My understanding is that they are locked into a constant chosen frequency (speed) and measured like that. In real operation, at low output, variable speed systems are more efficient than at high output. This can be a factor in choosing to use set backs, which can be counterproductive. My belief is that the manufacturers could fix that by allowing a set back recovery at less than 100% output with more sophisticated controls. My mini splits run very well on portable generator power. There is no start-up surge as seen with on-off units. They spool up very smoothly and slowly. If you have more than one system, you will never be totally out of heating or cooling. You might find this study useful. It was an eye-opener for me: Long-Term Monitoring of Mini-Split Ductless Heat Pumps in the Northeast Building America Report - 1407 December 2014 Kohta Ueno and Honorata Loomis

I am not going to tell you what to do, only point out some things that most people don't consider. If you think that minisplits will save money by conditioning part of a new modern, well-insulated structure (zoning), forget about it. Heat and humidity move from room to room much faster than inside-outside. Furthermore, the attic is gong to be sealed and the insulation at the roof line, it is a good place for ducts, HVAC equipment and your water heater. Electric furnace? Why not a heat pump if he is proposing ducts anyway? Get an energy rater to do some work for you. If you have a building with a very high performance envelope (insulation and air sealing) worrying about and spending a lot of money on high efficiency heating and cooling appliances might be misplaced. Capacity modulation is great, but when you move from 1:1 indoor:outdoor units to multiple indoor, you lose some. I think they all throttle down to about 30% on the compressor side. Installing two or more on the same outdoor unit limits the final low capacity. If you want to throttle back to the lowest capacity possible, you will use 1:1 systems. I don't know if anyone pays attention to this subtle fact. It can mean a big difference in how a house will perform. For example, putting all bedrooms on one of two outdoor units means that at night they might all throttle back to 30% before they start to cycle. If all three are installed on different compressor/outdoor units, they may not be able to modulate much at all.) After switching from ducted system to mini splits, I don't miss cold air blowing in the bathrooms. Heat would be another story entirely.