It's true for every Indonesian words, the plural-dobling is only optional, so "jeruk" can be orange or oranges, depending on the context...
If you want to translate a specific orange, (in English: I like the orange/that orange/this orange).
You would say:
Saya suka jeruk itu (= I like the/that orange).
Saya suka jeruknya (= I like the orange/his orange/her orange).
Saya suka jeruk ini (= I like this orange).
but, if you say "Saya suka jeruk", it's infinite, and you don't have a way to know if it's "I like an orange", or "I like oranges", as it's context dependant.
If you really want to mean "oranges" because it's not clear from the context, or very important to mention, you would say "Saya suka jeruk-jeruk".
Yes, "buah" means "fruit" and is used after a quantity for fruit or round things (according to my understanding).
And "seekor"is used after a quantity for animals;
For instance: I have 2 dogs. Saya punya 2 seekor anjing. You can also skip it: saya punya 2 anjing.
It's the way I learnt it. If it's not bahasa Indonesia yang bagus dan benar, please correct it.