I used an iPAQ running TomTom Navigator 2, before switching to COMAND - it'll look something like this if you decide to go with it . . .
Plenty of cheap ones available on eBay. You'll need a Compact Flash Jacket with at least a 128Mb Flash card + a GPS antenna to run it properly. Also works with Road Angel for providing the GPS signal if you ever decide to go with one of those.
I still have £50 worth of charging dash mount for the iPAQ, specifically for the W202, in case anybody is interested.
S.