I will be traveling to Germany next Monday, I have not bought the health insurance yet.
I come from Brazil (Brazilian). Is a travel insurance bought from WorldNomads valid as a health insurance for my entrance in Germany (Schengen visa)?
As a Brazilian citizen you don't need a Schengen visa (see Regulation 539/2001) and therefore don't need to prove you are insured to enter the Schengen area. If you would need a visa, you would have had to present a proof of insurance with your visa application and would not find yourself wondering about this shortly before departure.
The travel medical insurance requirement is defined in article 15 of the Schengen Visa code and then mentioned again in article 21 on the entry conditions that must be fulfilled to issue a visa. It's also one of the reasons to refuse a visa mentioned on the standard refusal form.
By contrast, it is nowhere to be found in the Schengen Borders code. In particular, article 5 of this code, on “Entry conditions for third-country nationals” mentions most of the conditions listed in article 21 of the visa code (valid travel document, purpose of stay, financial means…), except travel insurance. Lack of travel insurance is also absent from the standard refusal form in annex V. The Borders Code is the regulation that applies to third-country nationals who don't need a visa.
Of course, it can be beneficial to be insured for other reasons but it's not a legal requirement for a visa-free short-stay in the Schengen area.
It is correct that as a Brazilian citizen you are not required to have a Schengen Visa. It is also correct that regardless of this you should always have travel insurance in some form. If you have an accident, in most countries you will be required to cover any medical costs that arise from the accident, so it is advisable to carry such insurance. It also helps if you lose anything or your trip is delayed. The Schengen web site does provide a lot of good information: http://www.schengenvisainfo.com/who-needs-schengen-visa/