Yes, but they will need to be correctly chosen and placed so that they will function well. May or may not be cheaper than a ducted system. It will provide zone control so you can heat or cool only the rooms needed and they are very efficient, at least the inverter sets are. You need a professional experieced with installing these as they are quite different and they will need to be sized properly. I cannot guess at BTU capacity but from your discription you might be looking at one dual head unit for the suite and living rooms and a triple head unit for the smaller bedrooms.

"Why someone would buy Mitsubishi/Carrier when there is much cheaper option." Because Mitsubishi is an established brand with a good reputation - the others are...... well....... as a rule, an unknown. Note that you can buy a Mitsubishi Mr Cool DIY system for around $1500, but finding a reputable installer may be a challenge (because they can no longer bury the labor charge and/or can't mark up the equipment). The smallest Mr Cool, I believe is 12,000BTU which is too big for your needs. Mitsubishi make smaller units but not in the DIY series, so you end up getting screwed by the contractor again!

I wouldn't factor it in unless you plan to do the addition in the next year. Technology changes and ages. In the next 2-5 years they may have a different model with different, more efficient technology. It could also be cheaper. I would wait to buy a separate unit for the new addition after it is built and you can determine the size needed then. Also having 2 outdoor units is a huge benefit if one needs repair the other can cool part of the house.

Hi Nick, If installed and properly maintained, an air handler in the attic should not cause condensation issues. Between the mini-split and full AC system, it is going to come down whichever is going to make you most comfortable and which is best for your budget. Mini-split systems can be more efficient and don't require as much maintenance. We typically recommend mini-split systems for a garage because they are typically more cost effective among other reasons. If you have a contractor offering a full system for less than a mini split, I would recommend researching the company, the product they are offering, and what is included in the price a bit more. It is not often that a full system can be installed cheaper than a mini-split.