hi leforte,
Yeah the projector retrofit is not that easy

. Anyway, I am just trying to present a rough idea without getting into the nitty gritty details of what a projector retrofit is, which is essentially trying to put a projector into the original reflector housing.
Like you mentioned, it won't be as easy as just stripping off the reflector and putting in a projector. This is a good point that you have highlighted. And for E46 which is more complex would require professional help. I know a person who do projector retrofit and he does not recommend this solution for E46. Maybe for older and simpler cars.
But if anyone has retrofitted a projector into E46 without any problems, please do post it up as it would be a very interesting read.
Bosch projector = US$100 per pair
HID Kit = RM 1000 approx for a decent kit
Labour to retrofit = ???
Additional misc materials = ???
Aftermarket projector lights for FL E46 = RM 3000 approx.
You probably save slightly more than RM 1,000 in retrofit but the risk of someting going wrong is an important factor to consider. It will be very messy if you make an irreversible mistake in the retrofit.
While the most straight forward and quality assured method would be to get Bosch OEM Bi-Xenon projector lights which would set you back by around RM 5-6K for face lifted E46.
Bosch sells a xenon projector for non FL E46 which is cheaper and this makes it not very worthwhile to do a retrofit on non FL E46.
IMHO retrofitting a projector to an E46 is not a worthwhile thing to consider after factoring the pros and cons together with the cost of alternative aftermarket and Bosch projectors.