Audio is probably the most important, work on that, but if you're upgrading your video quality, upgrade things like lights or lenses before getting a new camera
Audio. I'd say invest in audio until you get something pretty good sounding (AT2020 for condenser mics, a Rode Mic for shotgun mics would be sufficent)
Then, when you are ready to invest, go for a good DSLR (Used D3300/D3200 are good) and get a cheap lighting setup, such as a couple of Umbrella lights (http://www.ebay.com/itm/391231607109). Learn how to use manual!