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I haven't done the swap, but did a lot of research into it.
Stock, 6HP21 shifts a little better than 6L45, but the real difference is in xHP - no tuning is available for the 6L45. Make sure the mechatronics are 03/2007 or newer so it's fully compatible with xHP.
Avoid the 6HP21 from 335i; the torque converter and gear ratios are different. Here in Australia, I could only find N52 6HP21 in late 2006 cars. In Europe, I believe it was also used from ~2010 until the end of production. Hopefully you can find one that's suitable.
Wow, parts are so much cheaper for you! Used 6HP21 costs around 5x higher here.
If you can't find 323/5/8i 6HP21 newer than 03/07, perhaps you could swap in the mechatronics from a newer one, for full xHP support.
You will most likely want to change the mechatronics sleeves to ensure good shift quality; it's especially important for xHP with increased line pressure. If you have the budget, replacing the shift solenoids should improve things a little further, but it's not strictly necessary.
Hope this helps.
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