If you buy a used car from a private party (I did) be sure to check to see if there are any liens against the car from a financial entity or for non payment of patentes (license plates) and/or multas (fines). You should be able to do this on line (ARBA in the province and ABL in Capital Federal).
The license plates actually come with the (used) car. New plates will not be issued. You'll see the license pl;ate numbers are etched on the windows of the car.
You can have all of the paper work done by a Gestor. The only thing I had to do on my own was drive to a place to get a paper that certified the serial number of the motor matched the number shown on the title. The gestor did everything else.
I was bale to get insurance (Maphre) on line through Banco Santander Rio. I already had an account.
PS: I did not have an Argentine driver's license when I bought the car, but I got one a week later with my US license and without taking a written or practical test, something that was (at least at the time) possible in the province but not in Capital Federal.
PS2: As far as i know, if you don't have a DNI your foreign driver's license is only "valid" as long as your tourist visa is valid. I'm not sure if that is extended by 90 days if you get a prorroga de permanencia (90 day extension) from migraciones. Others have posted that even with a DNI they have driven with a foreign license without any problem for much longer than 90 days.