Just some comments from my experience.
EBay is a wash with satnavs
Waze on the iPhone and android uses live information and is very effective
Many map apps now offer downloading to avoid bills on GSM and connection issues, some are even free !
Go for the largest screen size you can fit on the dash for safety.
Fit in the corner not the middle for road law.
I use the iPhone or iPad Mini for everything in the car now.
I even prefer it to the built in unit in the MB just delivered.
