I'll need some input from other people here!
1. If you are using iMovie 08, don't. As an iMovie 08 owner, you can get the previous version, iMovie HD, as a free download from Apple.
2. For this kind of work, you will need to export your audio track for processing. From memory this is quite straight forward in iMovie HD.
3. I have only ever played for any length of time with 3 tool sets:
- Free tool and plug-in I've found on the net
- Logic Studio's included plug-ins
- Waves plug-ins
This does, however, cover the spectrum. Now, trying to remove the kind of noise you are talking about will require pretty mean tools, and probably a lot of your time a patience. Regardless of plug-ins, I suspect that we're not looking at applying one or two plug-ins to the file and hitting process. You will probably need to crawl over this audio doing serious, semi-manual second by second massaging, using the best tools for each block of different acoustic messiness. Wind-gusts, for example, will require different treatment to background conversation will require different treatment to white or pink noise. Background hubbub will be hardest because it's obviously of the same spectrum and acoustic properties of the human speech you are wanting to differentiate it from.
As for choice of tools, I'm calling out for help here from people with broader experience. I've never had much joy with free tools on this kind of problem. The tools included in Logic Studio ($500) are okay, so using Soundtrack pro (included) I'd experiment with noise-print noise reduction, the other noise-reduction plug-in included that just has a series of sliders, side-chained frequency targeted gates, and manually drawing an amplitude envelope. Expect disappointment. You have a big "poo in" problem here. Now Waves plug-ins change the definition of expensive (I used someone else's). I think a full suit of plug-ins will set you back about $7000, so you'd want to know they'd do the job. Waves have some specific restoration/ noise reduction packages, costing in the thousands, that may be of particular use here (that I have not used). Broadly, Waves are superior to logic's plug-ins, both in terms of sonic quality and (probably) results. I'm still guessing it would require second-to-second examination of the audio choosing the best set of tools for each problem that is occurring. What do others think?