Personally I would leave it on the back. You put more stress on the driving wheels when towing, and as I said above you add a max of 80kg to the weight at the back when towing, and that is shared between the 2 rear wheels, so only about 40kg max on your spacesaver.
check out a tyre and wheel company on the internet called tyreleader, i got my steel rim from them £29.00 delivered and had a pretty good part worn fitted at a local tyre bay £20.00.
All fits under the boot floor nice and cosy. you need the following.
Opel Astra-J / Sports-Tourer 1.7CDTI (09/2012) 6.5 Jx16"
with 215 x 70x 16 tyre fitted. These fit the anatara.