A lot of good advice has already been given.
I recently upgraded from a Marantz PMD-660 to a Marantz PMD-671 (24-bit.) Both were modified by Doug Oadie. I got over
10 years of use out of the PMD-660 and was able to sell it - in great working condition - to a fellow filmaker. I expect to get several years of use out of the new one. Somewhere in the neighborhood of $600 for that setup. Very reasonable considering the amount of service/life.
I say that to say, think of audio like lenses... more of an investment. At least MUCH more than the camera itself.
You're looking at a couple/few years average until you want to pick up the latest high-rez thing coming down the pike, but lenses stay around, and so does sound gear (doesn't change much, thankfully.)
I shot my first film with the PMD-660 (which provides 48v phantom power, btw) and a studio condenser (Peavey Studio Pro M2 - $150) set to hypercardioid pickup pattern. I have absolutely no regrets about the sound I was able to capture; turned out great.
Near the end of filming, I got my first shotgun - a Rode NTG-3. Did a bunch of research and found multiple comparisons which put it favorably against the workhorse Sennheiser MKH-416. Excellent microphone.
To me those were fairly expensive items, but in the bigger filmmaking picture - that's relatively cheap stuff, AND they do a great job. You may have heard by now... you can film on the greatest camera, but pair the footage with poor sound and the whole thing "looks" bad.
Put audio somewhere toward (if not AT) the top of gear consideration.