I hate those kinds of windows, very popular in mobile homes unfortunately.
Make sure you have access to an outlet that can handle the load (no computers, tv's or other electronics on the same circuit as the a/c).
Don't center the unit in the window, put it to one edge, a 5k unit is small enough that you won't need a support, as long as you put the unit on the edge of the window. If you center it, you will need a support (outside) underneath to hold the weight (rather than the window holding it).
What you will need to do is install the window unit to the outside panel, and simply leave the inside panel up. I'd use plywood rather than the expandable panels, and screw the window in position, otherwise it is an easy entry point for thieves.
Make sure the unit angles down to the outside, or water will drip inside, calk around any cracks especially around the units frame, or you will get condensation dripping on the inside as well.
You can also install it to the inside panel, place the plywood on the outside panel, you can do this IF there is enough distance between the front on the unit and the start of it's intake fins.
Note that if you install this on a window that doesn't have an overhang, you WILL get water inside the window frame, no matter how well you caulk, seal and tape.