Second question first:
"Einer", "eines", "eins" are pronouns. They stand in for a noun.
Der Juli ist einer der heißesten Monate. (... one of the hottest months.)
Gibst du mir noch einen (akkusative) von den Keksen? (... one of the cookies)
Eins wollte ich dir schon immer mal sagen: Du bist super! (One thing I always wanted to tell you")
"Ein" is an indefinite article, it's used together with a noun.
Ein Mercedes ist ein Auto. (A Mercedes is a car)
Question 1:
The gender of the pronoun is defined by the gender of the noun it stands for. In this case it is part of the construction "eine[r|s] der Besten", so its gender needs to be whatever the gender of "die Besten" is. Which is ambiguous.
It would be easy if it said "der besten Autos" - Auto is sächlich (neutral), so it would be "eines der besten Autos". Or, with "Wagen": "einer der besten Wagen".
In this sentence though, you can't tell, so you can use either "eines" or "eins" if you e.g. think of "Autos" or "Modelle", or you can use "einer" if you e.g. think of "Wagen" or "Mercedes".
What actually doesn't play a role here is the gender of "W201" (which is male, because for some reason actual car models almost always are: der Käfer, der 230 D, der 911, der Corolla).
Charlotte ist eines der besten Mädchen in der Klasse. (because it's "das Mädchen")
Die "Gorch Fock" ist einer der bekanntesten Traditionssegler unter deutscher Flagge. (der Traditionssegler: traditional sailing ship)
(Der) W201 war eins der Besten von Mercedes.
It's still not great style, because it's still not entirely clear what basic set "der Besten" refers to. The best of what? Coupés? Autos? Fahrzeuge? It would make the sentence clearer if it defined that.
(Der) W201 war eines der besten Modelle von Mercedes.