Upgrade on a Statesman - for me would be to a newer Statesman. Unless you were looking to change vehicle type entirely to an SUV or something.

Not sure which cars you could get 3 seats into across the back, it's not something I've tried. Though I have seen a friend get 2 in the back of his Colt, so I've no doubt it'd be do-able.

The petrol thing could be something like worn injector hoses or fuel lines, especially if it's fairly faint rather than overwhelmingly strong when you get out of the car. Worth getting checked.