I have the TyreWeld kit from Halfords and the instructions don't say anything about jacking the car up. Also, the M doesn't have a jack with its mobility kit.
I have bought an E46 space saver wheel for longer journeys... I bought an E46 jack (on eBay) the top of which fits the Z4 jacking points perfectly. CJ made a good point the other day, it's worth having a spare wheel anyway, even if just to get you to the local dealer. If you are in no hurry you can keep an eye out for bargains, you should be able to get the wheel and jack pretty cheap :wink: