Purchase price + cost to buy + improvements + cost to sell = basis
Selling price - basis = cap gain and since you owned it for more than 1 year it is considered long term and it gets a better/lower tax rate than your ordinary income.
Since your only other income is SS benefits then unless the profit is more than $80K you will owe no taxes however it does count as income for the ACA credit.
Now for the Marketplace insurance ... if your estimated income is different from your actual income then you need to let the ACA know right away so the advance credit can be adjusted. It is possible this will force you to pay back some of the advance credit you got this year. Now if you wait to close until 2021 then you will be able to tell the ACA more precisely what your 2021 estimated income will be so the advance credit is calculated correctly.
