Necessary to buy round-trip ticket to qualify for tourist visa?

You will need a ticket leaving the country. It doesn't have to be a round ticket or a ticket back to your home country. It can be a bus ticket, plane ticket, boat ticket...just something showing that you're leaving Brazil.

If you're really not sure when you want to leave, my suggestion is to buy a one way ticket to Brazil and purchase a "throw away" bus ticket out of Brazil. This is what I did in the past and there weren't any problems. I can't remember the exact cities/towns I chose, but they were border towns of Brazil and Argentina.

The only small complication is that bus companies in Brazil require a CPF number (similar to a social security number) in order to purchase tickets. But there are 3rd party websites that offer to purchase the tickets for you without the CPF number, at a slightly higher price.

Overall, it's a pretty cheap solution and (in my opinion) the best thing to do if you're unsure about how long you're going to stay.

I should add that you will need a visa to the country that you have an onward ticket to (if a visa is required to visit that country)

Keep in mind that Americans now need a visa to visit Venezuela, I know some websites haven't updated this.