to gtkid2002. For LCD use something like 128x128 Nokia 6100 screen, its cheep and has a good support (
Interfacing Nokia 6100 color LCD to AVR controller).
You can also look at this project for some ideas:
xarias
Regarding Mega board. There is one more (cheaper) choice: Roboduino Mega which is Arduino Mega clone, but comes from china. With all in black it also looks better then Arduino Mega. You can buy directly from them very cheep with some help of google translate (
Roboduino Mega).
I am also making a custom device based on Mega board for my vehicle. I am considering to use RPM, fuel tank level, injector, speed signals as inputs. Additionally I am going to use temperature in and out sensors. I will log all the data into SD card and as for interface I am considering to use bluetooth serial link to do all the manipulations via windows mobile device wirelessly. Once I am done and if I will still have some interest I will continue by adding GPS + inertial sensors such as gyro, accelerometer.