Help with the confusions around 了
V+了+object requires the object to have specified quantity


No, it is wrong. 

The following sentences are all legitimate:

我买了苹果 = I bought apple (not mentioning how many)

我买了两个苹果 = I bought two apples (stating the quantity)

我买了酒 = I bought alcoholic drink (not mentioning how much, not even what kind) 

我买了三瓶白酒 = (I bought three bottles of white wine)


  http://www.cantonese.sheik.co.uk/dictionary/characters/460/
  
  [1] [aspect marker] indicating completed action
  
  [2] [final particle] indicating change of situation
  
  In all the examples above, 了 is a aspect marker that indicates the action (verb) is completed


~

我买了苹果了 = I bought apple 

我买了两个苹果了 = I bought two apples 

我买了酒了 = I bought alcoholic drink (not mentioning how much, not even what kind) 

我买了三瓶白酒了 = I bought three bottles of white wine


  In all four examples above, the second 了 at the end is a final particle. It serves to emphasize the fact and stress an affirmative tone. 
  
  Your teacher said it is colloquial because we do use final particles mainly in speech, but it doesn't mean we don't use it in writing. 


~

我去买苹果(了) = I go to buy apple

我去买酒(了)  = I go to buy wine

*You can add final particle 了 to the two examples above (了 stress an affirmative tone - seriously, I am going to buy apple/ wine)*

~

我买两个苹果 = I buy two apples

我买三瓶白酒 = I buy three bottles of wine 

You shouldn't add final particle 了 to the two examples above because they are plain factual statements