There's a simple trim line up, but be aware that although SE may sound like the top spec, it is in fact a very well equipped entry-level car - there is no lower spec. The best are available in BMW dealers and although you'll be paying a premium for this, they do have the widest selection and the best examples. Those offered through BMW Approved Used cars have a year's breakdown cover, 30 day exchange policy and a full history check.
Moving the car on shouldn't be much of a problem. Dealers - both official BMW outlets and independents - will be keen to have 3-Series Convertibles as stock, especially if they are well serviced, come with key options fitted and are low mileage.