If you buy a bike already prepped obviously its a bonus.
From my research the pro's of buying in S.A are no shipping costs from Australia (Around $2.600) or if coming from the U.S.A unless you specifically want to do Mexico and Central America you have a much greater distance to travel to get to S.A (obviously time and money the key here) which means more money and the cost of shipping across the Darien Gap, again costs vary but recent posts suggest by ship its about $900 approx for you and bike.
Also large bikes (650 +)are less common in S.A so attract a premium price, well above what you would pay for the same bike in the U.S. Often they have quite a few Km on the clock as well and often the title exchange will be dodgy as well.
Having said that often at the end of a trip its too expensive for the owner to ship back to country of origin and with time running out often bikes can be picked up for a bargain price if you are in the right place at the right time.
Those with limited time are more likely to fly in country and pick up a bike from another traveller.
Having looked at the pro's and cons of both options Im considering buying a new Honda cg 125 in Chile, about $1800 Aus new. Bulletproof and will cruise up to 100km depending on load. Known to be ok off road and great on fuel, easy to work on by any mechanic throughout S.A
Unless you have your heart set on a bigger bike or want to do serious off road riding Id give that a thought.
Just my 2 cents worth, only from what I have picked up with searches, no actual experience myself.