The M50 manifold can be cheaply sourced from chop shops if you look around. I have a friend who purchased one at RM150 a few months ago. Mine was purchased at RM200++ (can't remember already the exact pricing) about 2 months ago.
Depending on where you do the install, some places charge upwards of RM1000 (but this is inclusive of a M50 manifold already). My usual workshop charged me around RM400++ which includes labour, piping for the connecting joints (stainless steel) and all the necessary hoses (very tough hoses which is metal threaded inside). Time it took him to install was 1 full day which could have been faster if there weren't other cars around. The old 328 throttle body can be reused, just need to machine out a gasket to fit it.
If you car is stock with just the M50 manifold, your stock 328 brakes should keep up but it's always safer to upgrade them. Some people have gone with the E46 330i brakes which is plug and play. The other option would be to source for E38 4 pot OEM Brembo brakes. These are not plug and play and requires a certain amount of modifications to your spindle in order for it to fit. If done wrongly by an inexperienced mechanic it can be fatal (to your life
). The E38 4 pot brakes can be sourced for around RM800 onwards depending on your luck and how well you know the chop shop fellas.
There's a US M3 3.2 cams for sale right now in the classified section for RM2,700. If you have jalan in US, you can probably get it for half that price if you're lucky but that's not including the shipping and tax (which you can lari if you have jalan). Generally they're priced in the RM2k range onwards.
Manual conversion can be between RM4k to RM8k depending on which gearbox you use. I'm currently using a 325 gearbox with 3.15 rear diff (non-LSD) and my conversion cost me RM5k inclusive of labour. You can opt to go with the original ZF 328 gearbox which of course will cost you a bomb because it's very rare.
There's a lot of modifications which you can do for your car. It all depends on how deep your wallet is and how far you're willing to go. This is an almost endless route and be warned, once you start walking down the path of the dark side, there's no returning!
Slide, your car so power any modded 328 manual sure lose la. Unless supercharged then different story. The 328 auto gearbox is very very smooth and changes gear extremely fast if taken care off.
Just to give you guys something to ponder about, here's a list of modifications and their price estimates:-
1. PU Bushes - RM2400+ (SuperPro)
2. X-Brace - RM700++ (cheaper if you can source from US)
3. Sports suspension + absorber - RM3000++
4. Adjustable suspension - RM7000++
5. ARB - RM2000+++
This only covers part of the handling improvements you can do for your car. There's too many to list for engine so if you really want to do it, try the US BMW forums. They're full of information.