Surface cracking on the rubber bushes is quite normal once they age, it's no detriment. If the cracks on the rear bush start to develop into a tear, that's the time for replacement. You probably have a few years to go yet.
Not done wishbones on either Jazz, but I've fitted several of the cheap wishbones on other cars, without any problems.The Jazz wishbones can be had for as little as £20 each for a 'no-name' part, a few pounds more for a known branded factor part.
I've also refurbished old wishbones with genuine bushes. It can work out cheaper than a pattern wishbone, but obviously it's a bit more work, and a press makes fitting the rear bush a lot easier - the white metal bush outer shells are easily damaged by improvised means. The aftermarket Jazz wishbones are really cheap though, so it probably isn't worth the effort.