You'd be surprised how hard an a12 can go!
Your best bet is to drop into a/few local mechanics and have a chat about what recommendations as they will be able to guide you the best, especially price wise...
My mechanic starts at $80. i already know before i go there it will cost me 80 dollars before he looks at it.
ACL pistons are pretty generic...
I would use all your standard rods, crank flywheel etc. It all depends how much money you wanna spend. More money means a better a12.
I would recommend having the block bored and honed, a nice set of flat tops, have the crank, rods and pistons all balanced.
Have the flywheel machined and lightened a little.
Then you can do some head work, this is all good and well but it costs money! I spent a good grand having all my machining done which included everything being balanced, had the head shaved and also had the bottom end assembled.
you can have a standard cam reground to be a little bit more lumpy but id keep it mild, as a cam tha picks up at 5-7 grand would be terrible to drive on the streets...

A much cheaper alternative is to wait a little and hunt down a cheapish a14/15... an a14 has been known to rock up from time to time for between 450 and 600 dollars... Simple conversion if you have a late model cross member. Or modifying the engine mounts isn't too hard but from memory modding engine mounts will cause an engineers cert. but dont quote me.
If you ask around for brands put a few other maybe more specific posts up to the part you're looking for on here and the gurus should be able to help as im still learning... if anything compared to some of these guys i look like nothing! haha

When it comes time to buying gasket and what not go to Auto surplus in Mitcham
http://www.autosurplus.com.au/
Let them know Eddy from here sent you. They will sort you out.
They should pretty much be able to get you anything...
Otherwise you can contact member Bige, he builds a series race engines and will be able to answer any and all of your questions much better than me!!!
One thing to keep in mind, none of this is a cheap exercise always remember that!