I would say goto Mike and get him to put something in for you. You get a superior system for the money of the HK if you go aftermarket.
The HK system has an amp, and to my knowledge has the same number of speakers that the normal mini system does. Although as some are two / three way speakers I think they quote that the HK has 8 speakers.
By going aftermarket you dont run into problems with the standard system. I.E. aftermarket is easily extensible. If you had HK and wanted to change your headunit, then you have to get a HK enabler, but it is possible, just ask mike !
You could get something very nice for £500 aftermarket