Hi Noam,
Sound IS one of those fields where you get what you pay for; ie: a $500 mic is NOT a $1,000 one; there is a difference.
Before buying equipment, you might be better off hiring a sound person who already has their own stuff, and knows what they’re doing. This may, though, depend on how may days of shooting you have.
Running audio through a mixer will give you more control but, unless you’re doing all sit-down interviews, it may be hard to run a mixer and a camera at the same time. That said, I would recommend the Sound Devices 302 mixer, and the Sennheiser Evolution g3 wireless mics. Don’t get anything cheaper, in terms of the wireless mics. And don’t forget good headphones; Sony MDR-7506.
Best,
Dave J