Help with and unexpected purchase

So, I am in the position to have to replace it. After seeing what that car looked like and the fact that my husband walked away with only a slight neck ache that evening pretty much sold me on a Benz.
I am looking at an E350 and C350. I figured this mulligan will allow to get a car that we can get in and out of comfortably! In looking at the prices, I've found the E at $57 and c at $44 with the equipment and color that I want. Per Benz, the markup is 6% and they rebate 3, then the insurance company discount gives us about $2k. That puts it at roughly $4k off the bat.
In doing some research, it seems there is about a $4k diff in MSRP and Invoice, so that makes up what the dealer is offering. I know the cars have been 'price reduced' this year, but I'm wondering exactly what kind of offer is acceptable that I can stand firm on and the the dealership won't tell me to go away! Is $2k below invoice the standard? Also, what is the difference between the C and the E? Is it just vehicle size?
Sorry this is so long, but it's a story. They also have a trade in '10 E350 with 12k mi that I quickly got them down to $45,9 from 47,9 (prem pkg 1, sport pkg, nav, wheel pkg. MSRP was 53) If I can get them to $44 is that a good deal?
I want to make this purchase as quick and educated as possible. I figure if I'm prepared it gives them less room to do the sales thing, which I hate.
Again, sorry for being verbose. I appreciate whatever help you can give.
Kind regards,
Sqrl18


