Its as reliable as any other car. If its properly maintained, you shouldn't worry about it. Also, a lot depends on your luck as to how the previous owner took care of it. Although, 5K even in pounds seems a little low to me. I paid 10k EUR for 13' year with relatively low mileage.
What you should look for: stamped service history, proper mileage (check it with VIN) and open the oil cap on the engine, if it has yoghurt-like sludge underneath - steer clear of that car. Also make sure that the technical check-up (MOT) history is flawless. Other stuff are hard and/or almost impossible to spot if you have no experience with servicing the type of engine in the car.
What type of fuel is better, I'd say if you use it for shorter drivers (city, etc.) take the Petrol, otherwise the diesel.
Hopefully you find a RCZ that suits you. You'll absolutely love how it looks and how it drives.