Because you have an EU passport, your stepdaughter should be able to enter the UK under the freedom of movement rules as your family member. In UK terms, this means that she is entering under the implementing Immigration (European Economic Area) Regulations 2016. This is much less expensive than the "immigration rules" route and requires no proof of financial means, although your right to stay for longer than three months depends on being "qualified." One way to be qualified is to be employed, so you are all set in that department.
As a Brazilian citizen, your stepdaughter does not need a visa, but she can apply for an EEA family permit. If she does not apply for the permit, she can fly with you to the UK and present at the border the same evidence that she would have included in an EEA family permit application.
To underscore that she qualifies under the freedom of movement rules, here is Regulation 7(1):
7.—(1) In these Regulations, “family member” means, in relation to a person (“A”)—
(a) A’s spouse or civil partner;
(b) A’s direct descendants, or the direct descendants of A’s spouse or civil partner who are either—
(i) aged under 21; or
(ii) dependants of A, or of A’s spouse or civil partner;
(c) dependent direct relatives in A’s ascending line, or in that of A’s spouse or civil partner.
Your stepdaughter qualifies as your family member under 7(1)(b)(i) because she is the direct descendant of your spouse and she younger than 21 years.
What are the supporting documents?
With regard to being "qualified," you will not need to show this when you apply for the EEA family permit nor at the border, because you haven't yet been in the UK for three months. The basic documents you'll have to show are:
- proof of your EU nationality (passport or national ID card).
- proof of your stepdaughter's identity (passport).
- proof of your relationship (your marriage certificate and her birth certificate, showing your wife as her mother). This will need to be authenticated.
Do we need to meet the financial requirements ...
... and if so can my own income as a step-father be considered?
Yes. Another option would be to sponsor her under the immigration rules. In that case, your wife would be the sponsor, but I believe your entire family income would be considered. This is expensive, and if I were you I wouldn't do it; the main advantage would be to avoid the uncertainty around Brexit, but it seems unlikely that EU citizens in the UK will be kicked out. It would be too unsustainable from a political perspective.
Finally, can the child enter with a visitor Visa and apply in the UK?
For the freedom-of-movement route, yes, though it's better to claim the right of freedom of movement at the border rather than trying to enter as a visitor (for Brazilians, a visa is not necessary). After arriving in the UK, in any event, you may want to get her a residence card; after Brexit she will need whatever document they're issuing as evidence of "settled status."
For the immigration rules route, no. She must have a settlement visa before departing to the UK.
How does the UK's impending departure from the EU affect this? There are basically two possible deadlines, depending on whether the UK leaves the EU with or without a "deal." If there is a deal, your stepdaughter should be able to use the free movement regime to enter before the end of the "transition period" (currently expected to be the end of 2020 if I understand correctly). If there is no deal, then there is a chance that she won't be able to use the free movement regime after Brexit (23:00 UK time, March 29, 2019). But as is often said, "nothing is agreed until everything is agreed," so if you can move before the end of March, you should do so.