I use illustrator and photoshop, but I am not sure there are any trial or free versions you can use.
For the measurements and scale layouts, anything that lets you work in some kind of vector format is best.
For artwork, unless you are well versed in drawing with vector tools, it is best to use a program like photoshop to do your art, even if you are piecing together different pieces of art you are downloading. There are a lot of free programs out there to "paint" with, but every one of them will take some time to get familiar with and achieve the results you are after. In any case you should be able to save your vector layout in PDF and bring it into the art program you use as a raster image, just keep the DPI the same so the scale stays the same. For Control panels and marquees I work in 300 dpi so the print quality will be great, and the side art in 150 dpi which is still decent enough but not so massively huge that your computer will struggle with it. For reference, your screen is usually 72 or 96 dpi, so if it looks good on your screen, it will look good in print at even 150 dpi.