tyres ??

[P6 6ARY]

hi , i recently purchased my X5 and the front tyres had recently had 2 new bridgestone runflats fitted so are in an as new condition. the rears however will need replacing soon, my question is ..... i have read that runflats do not last and give a poorer ride quality so would it be adviseable to replace these with a good quality normal tyre ? i would then have to but a spare wheel from bmw (any idea of cost ?) i would then replace the fronts when required later down the line. is it going to cause me a problem running runflats on the front and not the rear ? any ideas/advice welcome

regards
gary