It can get real expensive man, you'll be up for an easy 10-15k$ by the time you pay for shipping and all that. Not to mention anything over 1000au$ you'll get taxed at customs.
If I were to do it all again, I'd look for a decent/clean bike, one you can ride and play with. Then bit by bit do it up. The killers are engine work and suspension, there is a good 5-10k$ in that alone.
I reckon a stock cylinder 4mm stroker would work well in the trails you can get some cylinders from the US ported and sent here. If you wanted a cylinder that you can re port for more power later on but not as good in the tighter trails, get a 421 serval, in stock form will be good for the trails, then you can re port for another 15-20hp but you'll lose some bottom end power.
Like anything how much have you got to spend ?
Ground up rebuilds are nice in thought but are to pricey and for something like a quad bike ya just gonna trash n thrash, is it worth it ?