Any Android phone will do, the defy mini just happens to be small, cheap and quite robust.
You dont need a SIM as you can download oruxmaps and any offline maps you want using your home wifi, if you have it, the phone just uses GPS when your riding.
You will need an SD card as the offline map files can be quite large and oruxmaps writes to the card, the Defy mini comes with a 2GB card which I have found OK at the moment.
The Defy mini does have a removable battery so you could carry a spare, I have done a couple of 4 hour rides recently with oruxmaps tracking the route and the phones battery has had about 35% left at the end.
There are a few mounting options for the phone as mentioned earlier in the thread.
Hope the above helps.