Standard OEM HID bulbs are 4800k (D2S I think), which gives a yellowish tinge to the light. You can upgrade to 6000k bulbs which gives a crisp white light, or even 8000k bulbs which are slightly bluish colour.
The other main beam bulbs are just halogens (H7 I think), only upgrade is to get Osram Nightbreakers (expensive though). LEDs for the main beams don't give any extra light, but will look whiter compared to halogens.
Fogs are halogen bulbs, same as main beam bulbs, so same as abve.
Side lights are 501s, so go for canbus error free leds.
Indicator bulbs can be upgraded to leds (easily if you have the orange lenses on the indicators).
All in, it won't cost too much and makes the older L322s look much better IMO.
Only down side is the 'disco style' light display when you first start the vehicle, as the LCM ecu does a test of the bulbs and the leds will flicker briefly (due to less voltage needed to illuminate the leds and they react faster than ordinary filament bulbs)