For the printer driving from a PC I'm very pleased with the RepetierHost software It uses the common Cura slicer, or may utilize some others. However, as I'm often using the remote controlled approach, I have the web driven OctoPrint firmware on my Raspberry Pi, connected to the printer.

For the free CAD solution I would recommend the DesignSpark Mechanical. It has a lot of professional features, easy to comprehend, has fast growing community, and quite good for a beginner overall. The SolidWorks is ultimate solution, but expensive.

I never heard of a 3D printing filament for printing metal casting moulds. Obviously, a typical 3D printer is a relatively low heat system. The closest available is liquid silicone printers, but they are quite different from regular ones (more complex cold liquid extruder is required). But, sure thing, you can use a 3D printed model to make an old school sand or silicone mould for casting. There are completely water soluble filaments available, used for printing pre-assembled things (i.e. a mechanical pen mechanism) in a dual extruder setups (another printer upgrade).

Upd: Just found that the cheap PLA is in fact burning out well enough to be used in place of the water soluble filament: http://www.instructables.com/id/How-To-Easy-Sand-Cast-3D-Printed-Objects/ You can define a zero infill and print the model's shell only (0.4 mm thick).