I think BMW make cars with Autogas capability.... The only thing you might struggle with is the new additional fuel tank. This usually goes in the boot (trunk) but I don't know if it'd fit in the Z4's.
There are loads of people who do the conversion who could advise you better. I'd think that the smaller engine might be more suitable if your goal is economy.
<span style='color:blue'>"So my BM makes me a flash bastard? Then I guess you're just keeping it real from the seat of your nova?"</span>
<span style='color:gray'>"...and the home of the brave..... is bravey 130bhp from a 7 litre engine? 100bhp/litre or don't bother building it!"</span>
<span style='color:red'>"Aaahh the UK; where every road is jammed, under construction, poorly maintained and unfairly policed, at least we don't get over-taxed for it........ HA HA HA!"</span>