Suing the developer due to smell and other issues

We purchased a gut-renovated wood frame townhouse in Crown Heights in November. Unfortunately we have two major problems ostensively related to the neighbor’s townhouse immediately to our left. The neighbors smoke cigarettes and marijuana indoors and the smell travels through the walls (?), vents and windows (her house is falling apart and she’s using plastic to cover a few of her windows). In addition, various food and toilet smell travels through the vents and stink up our house. In the evenings the smoke can be so bad that you would think that someone had just smoked inside our home. When we did the walk throughs prior to the purchase, we had smelled a faint smell of marijuana but we had been told that it was probably “the contractors smoking while working”, so much to our regret now, we didn’t investigate further (I know, dumb mistake).

As the smell has been there since we moved in, I’m confident the developer was aware of this issue but did not disclose it to us.

The second issue is that our basement flooded in December due to rain. The developer believes its be cause our neighbor (the same one) doesn’t have a proper system to take the rain water out away from her property and it caused my basement to flood. Through a slip by one of his contractors, we found out that this is likely related to an “issue known since the beginning of the project”.

I’m trying to address both of these issues with the developer who sold us the property but he’s being very slow and reactive. Although he’s telling us he’ll help us out, he’s not really taking initiative to proactively fix these issues. I constantly need to harass him to respond and they have yet to come up with sound actions to address the serious issue of smells.

I’m considering taking legal action against him as I believe he should have disclosed these issues to us prior to the purchase. I feel like I poured my life savings into a giant ashtray.

First, do you guys know what’s the most practical way I can go about bringing legal action against him? Second, has anyone ever heard of smells traveling between townhouses? It seems like somehow our HVAC system must be connected with their property but it has been extremely difficult to research any such occurrence online and even calling some HVAC pros hasn’t yielded any insights. They are even reluctant to come check it out.

I would appreciate any advice.

Note: I’m trying to be cordial with the neighbor and get her to assist me by fixing her property or not smoking in the house but I don’t have much leverage there.