2 for the doors, so thats one per side.

The ceiling lights, you need 3. two at the front and one in the rear.
lifespan, im not too sure, mine have lasted for more than 2 years so far. before that i had different colour, lasted alsmot the same time, probably still be ok. I changed them bcoz.....well i wanted a different colour....hahaha

eBay would be best go for these, be cheaper than autobarn n supercheap.

if you wanted bright ones, like i did, i looked for SMD LEDs or Flux LEDS, they were bright as.