I have the Sennheiser MKH 418s, and I can only talk from my own experience with this mic. Are you doing the location recording too? Or are you only going to be there to gather sfx?
alt text http://www.sennheiserusa.com/media/productImages/proMain/MKH418S_ProductPro.jpg
I'm in two minds about it. I love the sound of it and that it records dialogue as well as it does ambiences or some SFX. It's great that it can handle heat, humidity, cold and all that, but I find it very very noisy. Any recording that isn't too quiet is fine, but if I have to crank up the gain then the noise is just too much. It's to the point where I'm even wondering if it's damaged.
Are the weapons and planes and all that going to be shot and flown? If so you'll definitively need to purchase two in-line pads.
From what I hear, the mid is basically an MKH 416, so if you are there recording dialogue as well, then it can double up in it's use. When I decode the MS and only keep the mid, I find it quite quiet. Although as I write this, I'm remembering recording in the studio with the NT2A in one channel and the MKH418s' mid in the other channel and that made me really notice how much noisier it is.
This is where the problem seems to mainly be... If I only listen to the side (fig-8) then all the noise seems to be located there. This means that you probably can put some noise reduction on the sides whilst keeping the mid intact, and your quiet recordings should be clean enough.
If I had the cash, what I'd do is get the Schoeps MS stereo set...
alt text http://www.schoeps.de/IMG/products/product_pics/ms-stereo-set.gif
...and later on I'd upgrade to the Double MS. I'm not sure if it's just a matter of purchasing the second half or not.
Has anyone got the same experience with the MKH 418s? I really would like to know if it's just my mic or if it's normal.