I would personally go for a mini tablet rather than a GPS unit. You can use Google map or Waze offline now, just need to download them in advance. But you'll have a full functioning tablet for other stuff like Torque App, games, reading, etc etc.......
Some good read here --> http://mashtips.com/...ts-replace-gps/ (American price but gives you an idea about cost, lowest seems to fit your budget, just need to make sure they have car window adaptors before you buy them or design something to fit in the chubby). I remember seeing a member here using Velcro on an elastic strap looped around the chubby door, then added Velcro onto the tablet chasing.
Google map or waze offline --> http://mashtips.com/...d-waze-offline/
Found this in Ebay ---> http://www.ebay.co.uk/itm/UK-Seller-iRULU-Orange-7-inch-Quad-Core-2-0MP-Android-4-4-KitKat-16GB-Tablet-PC-/131443057724?hash=item1e9a9de43c:g:QdQAAOSw0JpV3XCe