Once the "gunk" has been injected, most tyre depots won't repair it. So you'll almost certainly need to buy a new tyre even for a minor repairable puncture.
So IMO the best solution is to buy the spare wheel carrier, if you don't already have one and you won't if you've ordered with 18" wheels. The club shop sells everything that you'll need.
As for the spare wheel, as it can't be an 18", a standard 17" or even 16" Transporter steel and tyre would do. Try your dealer or failing that Ebay! As has been said above, the rolling diameter of all three of these wheel/tyre combos are broadly the same. Also since the purpose of a spare wheel is as a temporary measure until you can get the puncture repaired or the tyre replaced, it will do just fine. Just remember to take it easy on the spare as tyres on the same axle should really be matched in terms of make, tread pattern, tread depth and ideally, the degree of wear. As you will be temporarily driving with both 18" and 17" or even 16" tyres, possibly of a different make and thread pattern on the same axle, it will almost certainly not behave as expected in an emergency situation especially under heavy braking on wet roads.
Better still change to a full set of identical 16" or 17" wheels and tyres including the spare. Higher side walls equates to a better ride quality.