microcontroller and some darlingtons or mosfets
microcontroller will let you custom write whatever kind of flashing sequence you want, without having to figure out different odd logic chips, timer chips, and other relics of the analog old school. sure there's a bit of a learning curve, you need to learn assembler for your chip (pic or arm or whatever), but once you do, the sky is the limit for different led light shows.
beefy transistors either darlingtons or mosfets will let you switch the large loads of high power leds.. powering it from a 12v system is no problem, you can use resistors and/or lm317 to "burn off" the excess voltage, or use a led driver (google for "buckpuck")