You have a fair amount of time. A bus is quite cheap as Manisha suggests. It does take up to 18 hours for a bus from BsAs to Puerto Iguazu. So you might need to take that into account. Though, the buses can be quite luxurious (eg flat lay seats).
No need for tours. There are buses between both sides from Puerto Iguazu to Foz and back. ANd buses to the park entrance. Its actually better - I saw a massive line of cars on entry to the park in Argentina and the bus went straight past them all.
Be aware that you may (it says Australian on your profile) need a visa to enter Brasil (at the moment we need a visa to enter, its about 250 AUD last time I checked). So take that into account. There are reports that you might not need it just to visit the waterfalls - but thats a risk you can only determine.
Flights within Argentina are expensive. Gringos get gouged on those flight prices. Booking ahead will help massively.