You should be able to check your luggage all the way through to EZE, and 1.5 hours should be more than adequate at IAD. You'll have to transfer from the A terminal to the C/D terminal, but that only takes about 20 minutes. You don't need to go through customs/immigration on the outbound leg, so that won't eat up your time.

You'll need to 'gate check' all of your carry-on larger than a newspaper for your JFK-IAD flight. You simply drop your bag by the stairs as you board the plane, then pick it up as you disembark -- so you'll have all your carry-ons for your IAD-EZE flight.

Hope this helps.