Check the roof loading limit of the 500L MPW if you are planning to put a roof box on there.
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Also, the 1.6 Multijet and 1.4 Turbo Petrol are more than powerful enough for the size of the car - gone are the days of sticking massive engines in cars just for the hell off it - Fiat can get 170bhp out of a 1.4 petrol so why would they need to fit a bigger engine - it's not just about engine size, it's also about bhp, torque & usability too.
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The days of huge engines are fading but ~100 BHP and 50 Kg (maybe it's 60 I don't remember) roof limit doesn't seem great to me for pulling 7 passengers and the big roof box that they'll require. I don't think the 500L is offered with a 170 BHP engine in the UK.
If all the passengers are occasional use and you only want a small roof box carefully loaded (and not a huge roof box for e.g. camping gear for 7) then it will be fine.