A BMW 340i Touring of around 2017 vintage with modest mileage is well within your grasp - sadly that's the only petrol 3 Series of this vintage with a six-cylinder engine. However as an all-rounder with better long-distance efficiency we'd push you towards the 330d or 335d models, which have smooth six-cylinder diesel engines with plenty of performance. Also check out the 4 Series with the same engines if a convertible is your bag - there aren't any comfort-focused six-cylinder hatchbacks, only the BMW M140i which is performance-focused. For comfort and refinement we'd also recommend trying out the Mercedes E-Class, which can be had within budget with either the E400/E450 petrol engine or the more commonly found E350d/E400d diesels. You might be looking at the previous generation version rather than the latest model for a budget of £30k - you'll need £35k for a low mileage latest model. However both versions can also be had in four-seat coupe or convertible forms.