South American is different from Europe both on and off the field. The passion might be the same but it is demonstrated in different ways. If you ever watch a Copa Libertadores match, especially the final, the game usually is delayed about 10-15 minutes due to all the smoke and fireworks. People literally live at times within the club. Unlike a lot of teams in Europe all teams are actually social clubs that have many members who pay a fee to use the facilities for many different things. Some even go to school or university at the club. To give you an example in Buenos Aires there are 24 professional clubs. In Montevideo there are 20 professional clubs. This is why the passion and hatred for and between each club runs so deep.
The relegation system in Argentina is that was to favor the big clubs. It was established to keep Boca Juniors from the second division in 1984 and is in usage ever since. In South America, especially in Argentina, relegation is death to clubs and supporters. When River Plate, team I support since childhood, was relegated there were riots all around the city.
As far as league differences Uruguay and Argentina are similar. You have the Apertura, opening stage, and Clausura, closing stage. In Brazil there is the national league and the first half of the year you play the state leagues which are mostly lower league clubs. They also all have different ways to qualify for the Libertadores, Champions League, and Sudamericana, Europa League.
Teams I would suggest are
Argentina: River Plate, Racing Club, San Lorenzo, Boca Juniors. Apart from Independiente, currently in second division, these are the biggest clubs and have some of the best youth intakes and facilities. Other Argentine teams with good facilities and youth intake are Velez, Lanus, Newells, Estudiantes.
Uruguay: Nacional and Pe?arol are the biggest clubs. Other interesting teams are Danubio and Defensor. Both are usually within the top of the league.
Brazil: Alot of big teams, however if its your first time I would go for: Sao Paulo, Corinthians, or Flamengo.
Teams I play with: River Plate and Nacional, Relative on board of directors.