U know yr stuff then.. Looking Fw the pics and video's..
A rooted android phone can be stripped to its bare essentials or run on linux. Boot time could be zero by leaving the device in sleep mode when you close the dash for e.g - most mobile devices battery life should last a number of days + have sleep mode func. so would be instant display from turn of the key and recharge on ignition
Interesting to see how you get the bits to interface and display.. The existing unit should have all the bits like you say though, just need to add wireless connectivity.
great plan mate