Speaking as a sound professional, and having been trained in video at the BBC, getting good sound is not just adding a Rode video mic. The mic amps in DSLRs are truly woeful.
Spend your money on a separate recorder (they are inexpensive these days). Even the built in mics on those will give way better results.
Your iPhone with an IK multimedia mic and a handclap for sync will yield better results than a dslr.
Hiring sound gear is cheap as chips.
Decent sound will separate your videos from the chaff much quicker than an f2.8 pro zoom will. Sacrilege to say on a photography forum, but it is very true.
Watch TV - more viewers complain about poor sound than the spherical aberration or coma or the poor depth of field. Get my point?