Hi Laura,
1. Yes, you must request a new Birth Certificate (long form) in some countries called Registration of Birth. This is the one that contains not only your full name, DOB, birthplace, but also the full names of both of your parents, their origins, etc. It must be submitted to the Consulado-Geral do Brasil for legalization within 180 days of the date of issue or it is not considered valid any longer. It also must be authenticated by the British Embassy or Consulate once here in Brazil (very important not to forget this).
2. The Criminal Record check in your case would be from the UK.
Regarding the overall cost of the marriage and visa process, it's really hard to say with any accuracy. Translators charge whatever the market will bear in any given part of Brazil, so if there is only one translator he can charge whatever he/she wants. In major cities where there is competition they all charge pretty much the same fees. I got my stuff translated in 2005 and it was R$200 per page back then in Belo Horizonte, so it's likely to be a bit more expensive now.
The Cartório fees I really can't say, you'd have to check with the specific Cartório they too charge different fees.
The VIPER process fees are Pedido de Permanência Definitiva R$102,00 this one you pay first of all when you apply. Later you will pay two others once the visa has been granted and you must register, they are Registro de Estrangeiro R$64,58 and Carteira de Identidade Estrangeiro de Primeira Via R$124,23. Then on top of that you're going to pay fees at the Cartório for notarizing your signature on documents (Reconhecimento da Firma por Autenticidade) and for making certified copies of any documents requiring copies. Your passport all pages (even blank) must be copied and that alone will cost a bit since they'll only copy the two open pages at one time so you're going to need around 18 separate copies for one 36 page passport, certified copies are around R$3 - 4 each in most Cartórios. Notarizing signatures is about R$12 - 13 each time (por Autentidade) and of course you'll need to register your signature at the Cartório first, can't remember the fee for that probably around R$6,00.
All told, I'd say for the marriage fees and the visa fees you should allow yourself a budget of R$2000 to 3000 just to be on the safe side. That would certainly cover everything.
