The middle part file is for stabilty of the two feets. Just insert them in the hexagon of the the stand parts, you can fix it with glue, but it's not required. You may print a second one for extra stability

150mm should work fine, added the 160mm part just in case you want that. but you can always change the height in the slicer

Printing at 15% infill is enough.

You can add some rubber pads at the underside, so the model don't moves when placing the guitar.