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The BMW GT has a brilliant boot but, if I were doing what you're doing, I'd get a Hyundai Genesis. Perhaps not as prestigious, but the same interior and boot space as the Benz. And there is a five-year free service deal and a five-year warranty, as well as talk of a guaranteed future value scheme.
The best option for you is probably to ditch the run-flats and fit regular radials. You can then run a regular spare and you could buy a jack from your local auto accessory store.