This seems clear.Sell and not pay capital gains because you've lived in it?

Still renting out, house has gained over $50k in equity and will only go up!

When the taxpayers demolished their existing principal residence, built a new home on the land, and sold it before ever occupying it, the Tax Court ruled the gain resulting from the sale of the never-occupied home did not qualify for exclusion under IRC Sec. 121 (Court case: Gates).Not sure if Gates applies here, because nothing would be added to the formerly residential land, only removed from it.

My duplex has gained about $50K in equity and I'm wondering if I should use that equity towards purchasing another property?

Hopefully you can gain access and get the property cleaned up.

how much do I owe for capital gains for fed and state?

When the taxpayers demolished their existing principal residence, built a new home on the land, and sold it before ever occupying it, the Tax Court ruled the gain resulting from the sale of the never-occupied home did not qualify for exclusion under IRC Sec. 121 (Court case: Gates).

Also, this home along with my other rental is depreciated, that means my real estate company, on paper nets zero dollars..... so unto $80,700 worth of stock market capital gains and dividends - are taxed at a 0% rate.  

After renting out my old home for almost 6 years, I am considering selling it now and curious about capital gain taxes as the house is now worth about $450,000.

(3) if we were to pull the trigger on this, we would likely want to do a Section 121 to preserve our equity gains, as much as possible, and pour it back towards the downpayment of the multi-family.