I've owned 2 N54s and one N55...
I will say that the N54 is one of the most potent engines I have ever seen in my life. With that being said; they are not reliable at all... from injector, waterpump, turbo and overall electrical issues, they are not the best used pickup if you want a car that will be lower maintanence. Personally, I think the N54 is the only reason to currently pickup an E90 as otherwise the cars are currently dinosaurs with quite outdated technology and ergonomics. If you decide to go that route; prepare for a potential headache.
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2 x N54 -> 1 x N55 -> 1 x S55-> 1 x B58
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