As for paying it down vs waiting-if you have enough to get a mortgage now and the down payment and the interest rates on the debt are low; then find a rental and buy it.  

Probably worth about 260k)Down payment: $100,000Interest rate: 6.75% (option to raise rate in 3 years but not down)Loan Term: 30 yearsBuyers monthly payments: $1,394.49When I crunch this into Chat GPT is says I am getting a 271.87% ROI.

I don’t think you said what school district it is. 

unless this property is unsalable on the open market or your selling it for far more than its worth this is a Pass full stop.. 4% is not even close to market rate for seller financing it should be 8 to 10%and if you need cash then you have factor that into their down payment.

DC is one of the lowest cap rates in the country.If you have a long time horizon, and have patience, with the areas strong rent growth you can end up with more cash flow here than a high cap rate market.

The rental has quite a bit but it's at a 2.875% rate that I wouldnt trade for this deal.

They are offering a 12-15 month 0% APR Credit Business lines, isn’t hard money usually a higher interest rate?

Their interest rates will be higher and they collect origination fees so the purchase price of the property is key and determining your ARV. 

Will it put you into the slow start of school season? 

$75k is best used for another house hack that way get a better rate and lower down payment meaning can put some of the money in rehab.