Most people are using cameras that require lenses. Lenses are going to be the most expensive investment.... But if you pick a brand (Canon, Panasonic, Sony) then you can use those lenses on most cameras... if they have the same mount.
Pick a brand, and then base your lenses around that.
I use the Panasonic GH4 which uses M 4/3 lens mounts. I also have an adapter for Canon lenses.
I still advocate a Canon Rebel T#i for starting out. You get a Super 35 sensor, can add custom picture styles like the ones from VisionColor and install Magic Lantern which gives you cropmarks, peaking, and other great features. As a bonus, it takes damn good stills too when you're doing the poster. I even used a T2i with just the kit lens to shoot an entire feature that now has one and a half million views on YouTube.
When you're starting out, you don't need 4K. You should be learning lighting, composition, and the basics of balancing ISO and f-stop. Basic camera stuff.
Your project will only look as good as it sounds, because "Sound is half of the experience"
If your film looks terrible but has great sound, people might just think it's your aesthetic.
If your film looks great and has bad sound, people will think you're an amateur. Sound is the first indicator to the industry that you know what you're doing.