"Did you guys buy insurance?"
You are right.
If we say in English "Did you buy insurance?", the speaker expects that listeners should have bought it. Although the Chinese question here is alright, a closer way would be to use a negative question, 你们没买保险吗？
Is it correct to use 有沒有 before a verb? My chinese teacher always makes a point that although many people say sentences like 你有去過中國嗎 that this is incorrect.
Your teacher does have a reason because this form implies that it is allowed to reply with 有/have + V, which is actually wrong in Mandarin (albeit it is alright to do so in English, Cantonese and other languages). However, as millions of people have been making questions like that for a hundred years, it is perhaps not very meaningful to insist on the "correct" form (such as 去過沒去過). Just be careful not to reply with 有/have + V (我有去過 would be wrong, 我去過 would be correct).
Does the Chinese sentence need "le"? I do not think it is the pass simple when i read it