I'm thinking 3 days Iguazu Falls- Puerto Iguazu

either 2 days/2 nights, or 2 days/3 nights, no more time needed.

And 4 days in Colonia Carlos Pellegrini (Does anyone know how to pan on how to get there?? I'm struggling...)

This does seem to be the problem. It seems logical as part of an overland trip to Iguazzu, using some combination of bus and taxi transport, and then fly back to Buenos Aires from Iguazzu at the end. What I'd try is make a lodging reservation in Colonia Carlos Pellegrini. And communicate with your hosts to find out how to reach the town and leave, explaining the need to reach Iguazzu.

The rest of your itinerary looks good. If you have to choose between a visit to Cordoba, or a visit to Salta, go for Salta, because that city and that region is so much better.

Taking weather into account you should start from Peru and continue via Bolivia to Chile and Argentina.

I agree. You should start in Peru and move south. Due to the weather.

Sept-Dec (four months) will be just enough to see the main spots and highlights in all four countries.

This is true, sort of, but not really true if you want to include Patagonia, which is split between Chile and Argentina. Really for purposes of a trip like this you should think of Patagonia as a separate country. Almost everyone who visits there needs to fly from Santiago or from Buenos Aires. Just doing southern Patagonia can take 3 weeks.

You also need to decide if you'll include any places in northern Peru (Kuelap is awesome), or if you'll just strike out heading south from Lima. And the 4 countries don't fit neatly together as a route, because of the Andes mountains. If you go south through the Atacama desert in Chile toward Santiago, you'll miss the great places to see in northern Argentina.

From where to start and how much time to dedicate to each country? We have around 3 to 4 months dedicated to the trip and prefer to start in September or October.

Like the other users suggested. Lima to Buenos Aires is a pretty standard route for long term travel in South America. Starting Peru in Sept would mean you would miss the rainy season for places like Machu Picchu.

Which are musts to include on our itinerary. Our main interests are culture, people, nature (love to include an experience in the jungle, visit lakes, volcanoes and beaches and also love sailing), temples, cuisine! We are travelling on a simple budget, nothing fancy.

Too many months, too long a journey to suggest a place. Grab a guide book and scour the net. Plenty of ideas and information out there. Unless you want to pay one of us to create an itinerary for you (we arent travel agents).

We are also toying with the idea of buying a motorbike or renting a car for the trip - is this doeable or are the distances too long and roads too rustic?

Doable sure. Enjoyable - probably not. Especially with traffic in major cities being an absolute nightmare. Plenty of buses ply the main routes and are dirt cheap and comfortable.

I have been flying to Argentina for almost 20 years now, every year. I have a place there. I am a US citizen. I have always flown one ways (booked with miles as United and American only charge you 1/2 the miles vs. a round trip.). I have never had a problem entering Argentina, but I hear others have (not common.) And up until recently, I never had a problem getting on the plane to fly there. HOWEVER, a few months ago, I was flying to Buenos Aires from Lima, Perú and they wouldn't let me board. Argentina was not the problem, but the airline was (Avianca.) Luckily, I was able to buy a "fake" ticket. It was nearly $2,000, BUT it was a fully refundable ticket. The agent knew exactly what I was doing. She apologized and said even if I had the paperwork for an international bus ticket or ferry ticket to Uruguay, then I could board. Anyway, I was nervous about getting my refund, but they refunded all my money a few days later. Some other countries do require proof of return- the Philippines comes to mind. Now I carry fake paperwork for a return trip. Just book it on airline that let's you cancel the reservation in 24 hours. Print out the reservation and then cancel it before you go. Highly unlikely they will ever check that the reservation is still valid. To be safe, I always have the fake reservation booked on a different airline. I always fly one ways everywhere, so that's is how I have worked it. No guarantees. Good luck.

It sounds to me as if you have almost answered your own question. For New Year, there simply is nowhere else like Rio. Reveillon on Copacabana Beach is in a league entirely of its own in terms of spectacle and atmosphere. Yes, it can be expensive, yes you will need to book a minimum stay but if the alternative is Buenos Aires, then there really isn’t much thinking required. Buenos Aires can’t compete and it can be just as muggy at that time of year, without the advantages of a beach to take the edge off the heat.

Do be aware that most coastal towns fill up at New Year. Salvador has a big reveillon and accomodation shoots up in price and requires minimum stays just like Rio.

There are as many trekking options in Brazil as there are in Argentina. There is absolutely no need to limit yourself to the coast, simply because you haven’t done any research. You mentioned visiting Bahia. Try googling Chapada Diamantina.

Buses are better in Brazil and yes sleeper services exist. What you need to consider though is that airfares, bought in advance are often a cheaper option for long hauls. If you were to combine Rio with Salvador then you would be better off flying. By the by there far more to each city then beaches. Each is a very individual, very different, cultual gem.

It’s called the White Party.

Really? Whilst it is true that it’s traditional to wear white, in all my years I have never heard of New Year being referred to as ‘a festa branca’. Reveillon, ano novo yes. Maybe that’s what they call it in the tourist brochures?

Non-stops from Miami, Houston, Atlanta, etc. to Buenos Aires. Both the domestic and international airports have non-stops to Bariloche.