Keep in mind that it is not only clock and date functions. It is any functions that you directly input through the mid, all of those functions will be lost. There is quite a bit. I figured the same as you when I gutted my e39 and installed an entire aftermarket system in it, the function of what I got was well above the form. I wont be taking the same approach with my e53 however, If I need nav, which it looks like I will, I will be doing a factory retrofit. I assume you had the standard mid in yours? Also, if I could say one thing about the dash kit you get, go with the Janus Designs one, they are hands down the best looking kit which retain the factory lines of the e38/9/53 cars. They are a little bit more expensive, but again IMO worth it. If you are creative enough, you can order a factory bmw kit and modify it to fit a double din...thats what I did with my e39.

good luck!