Hi Sebastien,
I was thinking $15k all up. Parts, labour, upgrades to other components (eg the engineer might want brakes the equivalent of what would be on the Subaru it came from) and engineering certification.
I think perhaps you underestimate how much work there actually is to do and how fiddly these things are. PLUS you then have to get all the engineering certificates and so on. It's also fair to suggest that the state you get it done in will vary the cost. Rates in SA are lower than they are here in the eastern states. Also bear in mind that if you get the work done in another state, but need to get it registered in SA, you'll need to get the car back to SA to get it engineered, and what do you do if they require more mods before passing it?
I know there was a business in Qld that advertised a drive in drive out price for Kombis, but I can't remember who it was. From memory it was about $10k, which I think included all parts, but you have to remember things are a lot more straightforward in a Kombi because there's more room. There's a lot more cutting and squeezing in a Beetle.
Whilst they are in different countries, here are two examples:
http://www.tsrengineconversions.co.uk/products.aspx?1=285&2=286
http://www.vanaru.com/index.htm
I know I'm sounding like a terrible naysayer, but these things aren't cheap, unless you can do the work yourself, and there's nothing worse than seeing a project started only to be abandoned because funds have run out.
It would also take a little while to get done, so you'd need to have access to a another car while it gets done.
One last negative thing, check whether you'd be able to get insurance on it. You're still quite young? They may object to you having a much more powerful motor in an older car. Best check these things before you start.
So, call your insurer first. Then call the VW places in SA and find out if they do conversions and what they would charge. If you don't have any luck there then start in Vic, and if no joy there then try Sydney. I know Production Automotive at Penrith do them, but I've no idea on the cost. If you get a couple of prices and you still want to do it, then go for it, at the end you'd have a very potent sleeper that would be a blast to drive.
Cheers
Craig