Debt vs deposit

1. Paying off debt faster will reduce your LVR loan to value ratio which means you can either borrow against equity you have built up or cross secure two properties together for one big loan and get a lower LVR across the two properties (not recommended to cross secure as if you sell one property you have security problems with the lender and you could lose both houses)
If you get the LVR < 80% you can take an equity loan against the property up to 80% and use the borrowed money as a deposit for the next property.
You could argue that the extra payments are a savings scheme and is evidence of savings as long as you agree to lower the repayments to allow for the next loan. The bank is interested in if you can afford the extra repayments for the next loan.

2. Your question is not really asked properly. So I am going to state the problem.
The 70k Savings is about your Financial Position
where as the 15k Debt is about cash flow.
If you owe 15k the bank looks at the repayment commitment on the 15k debt.
This is known by another name being Serviceability of the borrower.
Where as the 70k is the deposit however the banks lending program looks at the 15k debt at what ever interest rate and term and it calculates and reduces how much you can borrow not due to deposit LVR factor but more for a serviceability factor.
Another trap is if you have a credit limit on a credit card this will reduce the lending amount based on if the full debt was borrowed even if you owed only $1 on the credit card.