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If by lane assist you mean lane departure warning then yes, it must have KAFAS. And if it has KAFAS and lane departure warning, then possibly only FSC is needed. It could be that lane departure warning is included in driving assistant plus option (5AT), but ETK indicates both part numbers are needed for that car. I really don't know why. If willing to spend around 50€ just to try that, you can try ordering FSC only by the part number I indicated to your earlier.
Update:
After more peeking around ETK I see that pre-requirement for 8TH retrofit is navi professional and one of: Lane departure warning, Driving assistant, Driving assistant plus, Active cruise control+stop&go. So probably the info that I found on other forum that part number 65 50 2 184 753 is camera upgrade is wrong. I now think it actually is the price for speed limit FSC first-time creation.