We have a 3600sf house with 1800 on each level. Downstairs all open, upstairs 5 BR.
We installed a 36,000 BTU Chinese unit (Friederich) on the downstairs wall.
Ridiculously efficient, quiet, works well even when it's close to 32F outside. We use it in the shoulder months - Oct-Dec. and March-now.
VERY IMPORTANT - to remove all doubt about sizing, get an inverter/variable speed compressor unit and you can err on the side of too-big with no worries. Our unit, for example, is rated for 36,000 BTU (maybe a bit less), but will function down to about 10,000 BTU if there is a light load. That means that we had zero worry about over-sizing the unit, which oversize, during cooling, can cause short-cycling and a failure to de-humidify in one-speed compressor systems.
The unit automatically adjusts compressor speed and output fan speed to keep the room perfectly dehumidified and at whatever temp we set. Remarkable, really.
We don't use any separate system upstairs - it's somewhat cooler/hotter, but tolerably so.
In your situation, a minisplit in each room upstairs probably makes the most sense.
Regarding price, I got mine from a friend who works for an HVAC company - he got it for me at his cost, which was 1950, including a 50' lineset. I spent about 300 bucks on other parts, including outdoor pad and hardware to hang/run lines.
I installed it myself, which included hanging the evaporator unit on the wall, drilling a large hole in the floor (ran the lines inside the house and garage) and outside sill, leveling a spot on the ground for the compressor pad, running a 14g electrical line next to vac. lines to connect evap to compressor, and running a short line from panel to compressor (usually you'd use a disconnect box on outside wall). That all took about 4-6 hours.
I brought in a different HVAC friend who, for 150 bucks, connected the lines I had run to the compressor and the evap, and vacuumed the system to remove trapped moisture and check for leaks.
18 k sounds like a lot to me for what you're getting - Ithey really are very simple to install - particularly if you're blowing a hole in the outside wall and running the vac/14g lines down the side of the house. But that depends on where you live, and so on. In CT, when we lived in Simsbury, everything was marked up 30% as soon as the contractor crossed the town line. Ridiculous, but that's what it is.
EDIT - Be cautious about getting units with built-in resistance heating that kick on at low temps. That is equivalent to baseboard electric, which is very inefficient. If you plan to use them for your only source of heat in winter, then you need to look closely at cold weather performance of the unit at which you are looking.
