Backpacking South America

I plan on bumming around South America this summer/spring for about 2 or 3 months starting in April. I was wondering if someone has ever done this and can help me out. Or if someone is doing the same and we could meet up! So here's a few questions I have...

Is SA cheap like Asia or is it closer to Europe in terms of basic living costs?

Is accommodation in hostels easy to come by in most places or do I have do book in advance?

What is the transportation like? Do the cities have good public transport (bus or metro)? Are buses the cheapest and most efficient way to get from country to country? This along with accommodation is probably my greatest concern. I was also in Europe last summer and one thing I regretted not bringing was a skateboard, I don't mind walking but it just makes things way faster with a set of wheels. Would a skateboard be useful down there? Keep in mind I'll be backpacking so it's an extra burden to carry if I don't have a hostel to leave my bag in for the day.

Are there any specific countries or sites that I can't skip out on? Are there any that I can? I don't mind missing a few places if it means I can chill a little more and take things slow and appreciate my surroundings more.

Any safety tips?

I'll be in Cancun for about 10 days prior to going to SA. Should I just keep moving south from there and hit countries like Nicaragua and Costa Rica? I'm leaning more towards just flying from Mexico right into SA, any suggestions on which country I should fly into?

So yeah if you guys could throw any info my way that would be great. Thanks.