Hi there Mia,
Here are some thoughts, in no particular order...
You've asked about how to plan your time. I have travelled pretty extensively around the world and I honestly think the bus service in Argentina is the best I've come across. Distances here are long, so I'd recommend using the overnight buses to travel between the places you mention. The buses are comfortable and well-priced (nothing like the National Express!!), and you'll arrive at your destination refreshed. There are generally 3 levels of service. A 'semi-cama' is pretty basic. The seats don't recline that much - similar to economy class flights I suppose. The 'cama' services have far roomier seats which recline to about 30 degrees off horizontal. They are slightly more than the semi-cama, but definitely worth the difference. Lastly, there is the 'suite' class (aka 'ejecutivo' and probably other names). This has fully-reclining seats. The advantage in taking the overnight buses is that you're saving on hotel costs and not wasting valuable holiday time being stuck on a bus.
Yes, you can cross from Brazil to Argentina at the Iguazu falls.
If you're wanting to include BA, Salta and Mendoza then I'd go from BA to Mendoza then Salta. Salta to BA is a long journey, so you may decide that you're best flying this leg.
In Salta and Jujuy, I'd definitely recommend spending as much time as possible out in the surrounding area. Purmamarca and the nearby towns are very popular. Iruya is nice too. I have spent some time in the area and enjoyed it a lot, although it was more touristy than I'd previously thought. When I was there we hired a car simply to have the benefit of flexibility and freedom to do things at our own pace. Alternatively there are also plenty of local guides who can organise various excursions. I've heard good things about a guide called Angie, although I haven't personally met her. We're not allowed to post email address, but if you PM me I'd be happy to pass it on if it helps.
If you'd like to break up the BA - Mendoza journey I'd strongly recommend the area around Cordoba. The city itself has some interesting things to see, but the real highlight is the sierras to the north which are excellent for outdoor activities - think horse riding, hiking, swimming in spectacular rivers (not in August though!), paragliding etc... The area is well-known amongst Argentine tourists, but strangely undiscovered with international travellers. I'd like to be completely open and mention that I work in tourism in the area, but I'd be happy to answer any questions you have regardless of whether you're interested in what I do.
If you have any specific questions please ask - there are plenty of people here who can help.
Cheers,
Tristan
