I don't know which country you are from, but the process of certification (also called notarization) and authentication (which are not the same thing) are different for countries like Britain, US and Canada. So I will just use the words I know as a Canadian.
1. If you have been married before and divorced now, you must get the divorce certificate certified and then authenticated by the government (foreign affairs/global affairs). Why certified? Because you need to send the original copy, it's best make a photocopy and get that document certified as an original copy, so you can keep the original document. This same process applies to Police Records.
If you have not been married before, this process only applies to the Police Record. It is not required to get any other document authenticated.
If you are in the US, verify what I said above on this site:
2. If you are not married or never been married, you need to provide proof of this status and your current status. Being engaged doesn't change your single status, you are still single. I don't know what the process of this is like for Britain, but for the US and Canada, you write a 'Statutory Declaration of Marital Status' document and outline very specific information about yourself and your Moroccan partner. Failure to do so may invalid this document. There are many examples on the internet about this, so find a template and use that. Make sure it is one page.
For example, my document started with the header of my (full name) name and my parents name in the city I live, the province and then the country. All of this in sentence form... I then wrote that I declare the following: My Date of Birth and Birth Place, Citizen of Country with Citizenship Number and Passport Number, My Address and Mobile Phone Number, Single Status, Engaged to Partner's Name and Partner's Single Status, Partner's Citizenship Status, Moroccan ID Number and Passport Number. Partner's Address and Mobile Phone Number. And then you write that you declare this document is free of any duress and it is true as it would be if you were under oath... Signed by You, Printed Name and Dated.
Keep in mind, in Canada, this document MUST be signed and dated in front of a Notary or Lawyer. You cannot do this part before.
3: I would say 300 DH/P is a good rate, but you can find cheaper rates like 200 DH/P. Don't pay more than 300 and be wary of very cheap rates from someone young.
That is the official website of approved/certified translators specific to Rabat and Meknes. It's outlined pretty clearly even though it is in French. Name of translator, contact information, languages they can translate to and from. More details such as address and email is found if you click their names. Some translators tend to have experience with translating Mixed Marriage documents, so try to find someone that does and ask for help or verification of required documents.
I personally found a translator that charged 200 DH/P in Casablanca, but chose someone with experience even though that person charged more.
