For a 328:
One option is to buy:
2 used 330 trailing arms (2001-2005) - The 330 has a larger bearing (same as an M3), drive flange and axle splines.
2 Used output flanges for the diff (2003-2005) - You need these for the bigger axles but the flanges just pop out of the diff (easy to change)
2 Rear ABS sensors from an early 330 (2001) - Really early 330's had a ABS sensor that is a magnetic pickup and not a Hall pickup. A 328 has a magnetic pickup ABS system.
2 brand new axles from BMW from a 330 (2003-2005) - Might as go with new if you are looking for reliability. As for the Chinese axles, you are just rolling the dice.
This is what it takes to get the biggest and strongest axles of an OEM solution, for a non-M e46.