These are good questions and I wish you luck in your film journey.

I would try to get in contact with the live audio mixer for the event, and see if they will be able to record the stage microphone for you. Then you can get the recording from the sound guy. I think it will be difficult to get good audio of everything if you also have to be filming at the same time. If you are not able to get the recording of the audio from the stage microphone, you could use audio from a shotgun microphone placed near the speaker, but depending on how loud the venue is, you may or may not get usable audio.

Does your camera shoot 60fps? That should be plenty slow if you bring it into a 24fps timeline and slow the clips down to 40%.

I wouldn’t worry about 120fps, especially for a corporate/company event.

If possible, don’t use your phone. I’m not saying that phone videos are inherently bad, but they definitely are not as good as a DSLR under any circumstance.

Don’t break the bank buying new equipment for this one job. Your one camera should be fine for a highlight video and you will figure out what equipment suits you the best as you go.

Now, as far as charging goes, this is where it gets complicated. Since you don’t have any experience you can’t charge very much. I think that you could confidently charge $200 for the shoot day and then another $150 for a short couple minute edit. As far as driving goes, I’m not familiar with how far that distance is, but you could suggest something like $15/hr while driving and the flare rate for the rest of the shoot and edit.

Hope this helps and feel free to ask any more questions. I am happy to answer.