I was in the same position. Bought an e92 w/o navigation since i never used it in my previous car. Turns out that I like having a bright screen in there too.
After realizing the price of retrofitting a navi, i considered the dynavin. From the reviews ive read, its hit or miss. And its also a pain in the ass once youre modifying intricate systems w/non-oem parts.
I've decided on getting either an ipad or ipad mini and a mount and am going to use that as my entertainment system/center. Ive got netflix and spotify on there and there are plenty of navi apps to give you turn by turn directions and its all touch screen.