Like I said when I answered you in PM the ideal would be for the community to pool the funding and allow HTC to hire recordists on site. I could spend $30k just getting to England for Tankfest, but there are tanks right here in America that you can drive, shoot, and record.
I did some preliminary work by having gone through this already, and hiring a recordist is the best way. I have the same equipment that the pros do, but travel is not cheap and I'm really busy these days. FMOD really makes this easy once the sounds are collected, so I wouldn't be adding much to that process. I can do the work with what I have already, but it would always be lacking something and that would nag on my perfectionist side. I'm going to do it anyway because I love a challenge, but what AH3 really deserves is quality and I know there are going to be gaps. Otherwise, to get to completion it is going to take time.
(EDIT) This should help:
http://www.drivetanks.com/tanks-tracks/They offer packages that will lower the cost a little. A quality recordist goes for $650-1000 a day; double if you need video. There are recordists that specialize in aviation. And there may be collections out there, but they probably would not let go of a copyright. A recordist working the way I suggest would be contracted and the copyright established ahead of time as the property of the parent (HTC).