How should I approach the seller?

So accessing the equity through a HELOC would be easiest by keeping them in your name.You can go with a traditional refi approach. 

It has a sales comparison approach value of 150k in the current condition in 2018.

It was deemed not habitable by every lender I approached (even though it did have a bathroom and kitchen).

Check with your lease and State laws but I attempt to take a proactive approach to avoid insufficient notice:Proactive - at least 60 days before the lease is set to expire I sent out a "renewal rate offer letter." 

I've tried the usual approaches, including "pay for the place you live before you pay to light the place you live" with no success. 

Any help with approaching this topic is greatly appreciated!

Here're additional guiding posts for you and food for thought: https://www.biggerpockets.com/member-blogs/10850/87253-should-i-scale-my-investment-from-single-family-homes-to-multifamilyhttps://www.biggerpockets.com/member-blogs/10850/86621-six-steps-approach-to-getting-started-in-real-estate https://www.biggerpockets.com/member-blogs/10850/85836-should-i-buy-a-turn-key-property-or-invest-in-a-syndication

If he’s in danger of losing the house I want to approach him and propose a win win strategy where I help him financially, and I end up with the house free and clear, or with him as a tenant, and me and the owner in the long run.

How do I approach the negotiation of the additional costs?