The cleanness will not hurt, most lighting in cars is just straight from the battery. But do expect a little bit of difference in brightness when the engine is running because the voltage is higher.
Fet to use = fet that suits the job. So probably:
- Logic level
- can handle the current (including thermal) although with just some leds it's not that much
- can handle 15V Vds
- has a footprint you like
- is available (enough)
- cheapest
