Halogen to HID headlights
 

My 04 530i has Halogen headlight Assembly. Will the HID fit my car?

It will fit but it will not work. You have to add a few modules and code for it to actually work. It's a lot of work and not worth the time to me.

any good idea for replacement headlights that’s some what better then what I have ?

Maybe I misread what you were asking. Are you asking to change you assemblies to factory HIDs or change just the bulbs. If changing just bulbs then you can find HIDs and LEDs for replacement bulbs and it's and easy swap.

TBH, I think you just need to code your LMA and let it know they are Xenons. I say that because my car came factory with XENONS and for some reason the previous owner changed them for the halogen housings (don't know, don't ask) and they worked just fine, the plug was the same and the car had no idea they were the wrong headlights. Last year I got a pair of new Xenon headlights since I do a lot of night driving and I could really appreciate the xenons. Swapped them both in about 30 minutes and everything is working perfectly. There is a section in the LMA module that you'll need to change and let the car now it now has XENONS and not halogens via coding but it's super easy and shouldn't take more than 5 minutes using NCSEXPERT and NCSDUMMY.
Also make sure you buy the Xenon ballasts since they don't usually come when you buy the headlights.