Hello!! I'm on the same page as you but I'm going OEM all the way... Haven't found any information about how to make this happen on the F25, just read some guys that successfully did it
You could buy a "retrofit kit" from Patryk at Bimmer Tech or purchase it on ebay.. It includes the CIC (Navi), the CID (wider 8.8" HD screen) and the usb for the glove box to upload music to the HD Drive of the unit...
Yesterday I tried to do this CIC/Navigation retrofit on my car with no success, so I tried to use the HD screen with the non-navigation unit and it didn't work properly.. But I did not tried to use the Navi with the smaller screen, I could try this and tell you if it works.. However the screen installation is really straight forward and could be done in no more than 15 minutes so my advice would be to get the HD screen as well, it looks beautiful and it's around $200 usd on ebay if purchased separately
And yes, you could get it updated with US maps, but you need an FSC activation code and programming for your unit, as each unit is linked with the VIN number of the vehicle, so you have 2 options:
a) Go to this link and learn everything there is to know about E-sys, building the proper cable and how to do all it by yourself.. You could even do a lot of cool mods like enabling Video in Motion, folding mirrors, etc etc -- Cheaper Option
http://www.bimmerfest.com/forums/showthread.php?t=619819
b) Hire someone like Patryk that has all the knowledge and he will make it happen for you, he even sells the "Package Deal" including all the hardware and software coding needed -- Expensive Option
http://www.bimmer-tech.net/
I have read a lot of great experiences with Patryk and I'm confident that you'll be in good hands with him... However, for me the price difference was around 700 usd + an estimate of 500 usd on taxes, so a total of $1,200 usd pretty much made my mind, so I went for the harder/cheaper route
Finally I just wanted to give you a heads up that I've encountered an issue when trying to fit the Navigation unit on the dashboard, it does not fit with the Equipment Mount that my car came with, on this diagram it's the number 3 part and my P/N is 51459193777, there are 2 other options for this part: 51459193778 and 51459193776. I've yet to figure out which one works for me
http://www.realoem.com/bmw/showparts.do?model=WY31&mospid=52632&btnr=51_8190&hg=51&fg=30
You could go to the thread I've started about this and see the pictures I've included over there.. If we add efforts maybe we could get this figured out sooner.. I have already posted a DIY guide to do this swap
Here's the link
http://www.bimmerfest.com/forums/showthread.php?t=624682