] Would I be correct in assuming the gossiping Lucas' wrote and told Charlotte and Mr Collins?

I think that's the most likely scenario. In his letter to Mr Bennet, Mr Collins refers to the Elizabeth-Darcy gossip and adds, "of which we have been advertised by the same authority." This must mean that the same letter that told them of Jane's engagement to Bingley also mentioned the possibility of Elizabeth marrying Darcy.

] I'm just trying to piece in who would be the Lucas' source of intelligence, Aunt Phillips?

Do you mean about Elizabeth & Darcy? There was no engagement at that point, so Mrs Phillips wouldn't have known anything. In fact, Darcy was in London and not expected back immediately. Elizabeth actually reasons that the local gossips simply decided that Jane's favorite sister would likely marry Bingley's best friend.

I suspect that the gossip started as a "Wouldn't it be wonderful for the Bennets if some wealthy friend of Mr Bingley married one of Jane's sisters," then changed to "Do you think that Mr Darcy is interested in Elizabeth Bennet?" and ended up as "Did you hear that Mr Darcy is planning to propose to Elizabeth Bennet?"