It's true that's always better musicians over VST's, samples or MIDI. The problem is that where I live it's really difficult get them, I studied music production in college and I know a lot of great musicians, but when I got a project, they're always busy (unless is financed). So I wanted to know some virtual instruments as last resource (right now I'm working on a score of a short for a contest, and they gave me only 2 days to score it and without financing... I'm a drummer and a keyboardist so I recorded them myself and some other instruments like a bass or a sax I'm using Kontakt and I'm really amazed of the fidelity of it)

Get studio monitors rather than just speakers (like the KRK RockIt), an audio interface with at least 2 inputs (Focusrite have great ones) and a condenser microphone (like the AKG c414) with a pop filter and some noise pads are good for quick foleys or VO's.
Other things like a high end mouse (like a Razor) or a drawing tablet (like a Wacom Intuos) are useful too to improve your workflow.
At least, is always good to have a color calibrator (like ColorMunki) near for quick screen calibrations.

1) free softwares I recommend DaVinci Resolve or HitFilm (or, if you will do more than one short or project, get the Adobe CC Suite, look for some webpages of filmmaking for a student discount)
2)NO... I do everything on a PC (because I prefer it), but depending on the software, you'll need a powerful one (check the requirements of the software you'll get)
3) Almost every filmmaker will tell you that the equipment is the least important thing (and it's true), more importantly is the experience or the capacity of your crew (a good DP will make great shots even when you only have a smartphone, same for the sound)
4) Make a good pre-production and trust your team.
5) Watch references of how you want your film to look like (for example, if you want a film noir, watch film noir movies with special attention) and do the research (is incredible how YouTube have tutorials on how to achieve looks for everything in post production). For sound, in few words, learn perfectly how equalization, compression, reverb and panning works.

I have some written that I can't film because of lack of resources, If you want I can give you the rights to film it.
If you're interested email me anubiz.creator@gmail.com

Find your way through. As an actor myself I worked with a lot of them, and everyone has it's own path to get in the characters and learn their lines.
For example, I'm more an improviser but when the director needs the scene with the exact dialogue, work and practice with the other actors before going on set, and I also set "cue points" in the script (highlight specific words on the script in the lines of your partners so you can learn which dialogue you have to say and when)
If the director isn't too strict about the dialogue, work directly with him, ask him if you think your character would improve if you do an action or a dialogue differently than the script.
If you can improvise, more than the lines learn the story arc of each scene (if the character needs to go to point B from A, how he would do that? how would you do that?)

Use a multiband compressor (as an expander); focus your frequencies on the voices of the characters with a low Q and a threshold about 1 to 3 dB over the "hubbub" with a ratio 1:3 approximately.
You can do the same with an EQ in series with a Compressor, it depends what equipment or plug-ins you have.
The best one for me is the iZotope Ozone, but I heard that the iZotope RX is the best for sound recovering and sound for filmmaking.

About microphones, you have good equipment; at the location be sure that the place hasn't too much reverb (record better near curtains or some kind of fabric), place the microphone as near as possible to the actors with a gain input of -12 to -6 dB for dynamic range (the sound engineer will thank you), and last but not least, ask your actors to speak a lot louder than they usually do.