Go as large as you can afford, and it doesn't really matter if it doesn't have goto as most dobs can be fitted with pushto systems at a fairly low cost these days and in most cases pushto is faster and lighter than factory goto. Remember that alt az goto systems, like on that dob, will not track objects with accuracy the way an equatorial system will, what they can do is find things - but pushto systems do the same thing. Goto will add complexity, and weight, and motors and controllers are usually the parts that need attention down the track.
Stick some encoders on the 12" and set it up with Sky Safari Pro and a wifi device like Astro Devices Nexus (they also sell the encoders and brackets now), and you will have a Pushto system.
PS, Shop around, check out Andrews in Sydney located here:
http://www.andrewscom.com.au/site-section-10.htm
go to the Skywatcher page and scroll down to your scopes, and find out what each supplier will throw into the deal and how much they will charge from shipping. Being in Tasmania that could be an important cost differential.
You can also get a good 12" dob for much less money if you check out Bintel's Dobs, and they have a shop located in Melbourne. Check out the specs for both, resolution, limiting magnitude etc, there won't be much difference.
http://www.bintel.com.au/Telescopes/...oductview.aspx