I'd have to say that keeping/replacing the 530d for work and buying a fun / weekend only car will be a better option, why compromise your fun car with having to be up to the daily commute.
Some options...
katerham superlight
westfield with a bike engine (megabusa ? )
turbo'd MX-5
radical
something from Quantum (nice fiesta based kit cars with 2ltr XR-4i engines)
elise
or a classic car.... MG RV-8, MG C, Lotus elan
I guess it depends how happy you are to get your hands greasy
HT