Well I really think it depends on the type of person you are. This is a little trick I use every time I'm unsure if I can go without something.
1) Take a coin and assign a heads/tails value to with nav, without nav.
2) Flip coin.
3) If whatever side the coin falls on makes you "relieved," then that's what you truly want. But if instead you get an "aww man" feeling, then you're just lying to yourself because you want the car.
On a side note, I think at this point $55k is too much to pay for a used M3. There are a relatively decent amount of new 08's left that are better-optioned for about 50-55. Might take some searching around but they're out there. I would try to look for one of those rather than jumping on this one.
P.S. If you don't get #4 and #5 right away, just ignore them.