Well, if you're only tracking and just need preamps, you COULD go with an Onyx 800R, and work with the 1640 as your main mixer/centerpiece. You'd have some pretty good choices there, too. The 800R has an MS input setup on the first two inputs, as well as some line/mic switches on 7 & 8, I think.

For monitoring back and doing a 2-bus to a CDr or something, you could return the 800r's outputs through the six aux in and 2 tape inputs. It'd be less than ideal of course, but a lot less $$ than other solutions...

I agree with alot of the prior post from JoeH.
I just thought I'd put my 2 cents in..
I did some mobile work for 2 or 3 months, what a learning exp.
IMO try to stay all rackmount, no matter what. It makes for a quick in and out. And when it's "all the sudden" pouring down rain, at 2am and you need to load up and get home. Time is VERY important.

O.K.
When I'm recording I don't use any EQ. So if you can do the same, you will not need the EQ on a compact mixer. Also I found that having a mixer was not easily rackmounted. Esp a 24 channel mixer. So I say ditch the compact mixers, no matter weather it's a 16 ch. or a 24 ch. You really dont need one and must like JoeH suggested, a Mackie 800R would work fine.
Now as far as price. You could buy the Mackie 800R's one at a time. And that way you still have a use for your wallet. :lol:
Back to serious again...If you bought 3 of the Mackie 800R's then you'd have 24 channel's for mobile use.
Your next concern would be bringing the mix home to mix down and add some EQ and reverb and whatever else. Then you would need a board. I'm not trying to get you to buy all this gear. I'm just saying I have done as you are dicussing doing and that's where I'm coming from.
Also the Studio Projects 828 is another option, in place of the Mackie 800R.
I have a 24 track MDM also, and this is how I plan to do mine in the near future.
Just throwing out some ideas and trying to share some "mobile" audio concerns.

StevenColbert wrote: Also the Studio Projects 828 is another option, in place of the Mackie 800R.

The nice thing about the SP828 is that it is a rudimentary mixer. Nothing fancy, just level and pan controls, but you can chain three of them together for 24 track capability. I've got three of them in my HD24 rack, and they work very nicely with it.

The Yamaha 01V96 mixer and a Behringer ADA8000 would give you 20 mic channels and 4 line level ins. Add another ADA8000 and you get 28 mic chans plus 4 line ins. You would need the MY16-AT option card on the mixer.

I use an 01V96 into an HD24 via 3 ADAT lightpipes in the field and it works just fine.