I would say: 何を買うべきか or 買えばいいのかわかりません。 Because I think this sentence means "I don't know what I should/would buy". *Depending on the situation, you can use 2) and 3), too. Espero que te ayude.

1) 何を買いますか分かりません。 grammatically wrong, but i understand what you try to say. 2) 何を買うか分かりません。 correct 3) 何を買うのか分かりません。 it is sounds like stupid cuz as if somebody told you what you need to buy, but you just forgot lol 4)何を買うのですか分かりません。 grammatically wrong, and little bit doesn't make sense 5) 何を買う分かりません。 grammatically wrong, but i understand what you try to say. 6) 何を買います分かりません。 grammatically wrong, but i understand what you try to say. 7) 何を買いますかが分かりません。 grammatically wrong, but i understand what you try to say. 8) 何を買うかが分かりません。 the same as 2) it means i dont know what i need to buy this is also sounds weird, but grammatically correct

Many of those would work, I think, but you must always include the particle か, as it turns the phrase before it into a relative clause. I think generally you'll find that the verb in the relative clause will be written in the dictionary form. So, instead of 何を買いますか, you should write 何を買うか. Whether it is required to so, I am not entirely certain, but the best example of all of these, based on my experience, would be 何を買うか分かりません.