Need advise to buy an IP, what should I buy, Old or New?

When you look at building depreciation on the 2 new 750k IP's you need to look at what the construction cost was as this can be depreciated as a building write down over 40 years . at 2.5% per year. (you can't depreciate Land cost). Also note depreciation is subtracted from your cost base each year you claim it. This means in layman terms – increased captial gain in the future – increased capital gains tax.

Here comes some of the maths
New Building
Building cost estimate say $200,000 as a guess – You need to know this amount to do this.
2 x 200k = 400k building cost
depreciation per year = 400k * 2.5% = 10,000 a year as depreciation expense.
Loan of 750k based on 100% borrowing . interest of say 7.5% per year = $56,000 in interest p/a
Rate and insurance guessimate of $2500 p/a
Possible Strata charges guestimate $1000 p/a
Total expenses = $59,500 out of your pocket and $10,000 depreciation = $69,500
Total Income at 700 per week * 52 = $36,400 (may have real estate commission
Nett Property Income/ Loss = $36400 – 69,500 = -33,100
(please be aware that this is added back if dealing with Centrelink or Family office for payment calculation for income deeming)
(23,100 out of your pocket after adding rent )
Old property
Building cost estimate Hard to know depends on age of building Quantity Surveyor can help with this.
Fitting depreciation per year on renovation – Hard to know  Quantity Surveyor can help with this.
Loan of 400k based on 100% borrowing . interest of say 7.5% per year = $30,000 in interest p/a
Rate and insurance guessimate of $2500 p/a

Total expenses = $32,500 out of your pocket and unknown depreciation on renovation = $32,500
Total Income at 400 per week * 52 = $20,800 (may have real estate commission to pay)
Nett Property Income/ Loss = $20,800 -$32,500 – = – $11,700 (Loss)
($11,700 out of your pocket after adding rental income)

Tax Benefit
Ownership Structure 120k p/a 100% ownership
any income over 80,000 is taxed at 40% till $180,000
New
Loss of -33,100 so tax saved = 33100* .40 = $13240
So you lose $23100 to get back $13240 each year
Old
Loss of $11,700 so tax saved = 11,700* .40 = $4680
So you lose $11,700 to get back $4680 each year

With the new option you are in a riskier situation
You have more interest rate increase Risk
You are affected more if rates rise and have to find more after tax money to fund cash flow shortfall.
Lets assume a 1% interest rate increase.

Building cost estimate say $200,000 as a guess – You need to know this amount to do this.
2 x 200k = 400k building cost
depreciation per year = 400k * 2.5% = 10,000 a year as depreciation expense.
Loan of 750k based on 100% borrowing . interest of say 8.5% per year = $63,750 in interest p/a
Rate and insurance guessimate of $2500 p/a
Possible Strata charges guestimate $1000 p/a
Total expenses = $67250 out of your pocket and $10,000 depreciation = $77250
Total Income at 700 per week * 52 = $36,400 (may have real estate commission
Nett Property Income/ Loss = $36400 – 77250 = -40,850
(please be aware that this is added back if dealing with Centrelink or Family office for payment calculation for income deeming)
# any loss over 40k will actually be lower tax deduction at 30% due to lower tax bracket !
at 11.5% interest($53,350 out of your pocket after adding rent ) to claim back 40% of 63,350 = $ 23005 #
at 10.5% interest($45,850 out of your pocket after adding rent ) to claim back 40% of 55,850 = $ 20775 #
at 9.5% interest ($38,850 out of your pocket after adding rent ) to claim back 40% of 48,850 = $ 18505 #
at 8.5% interest ($30,850 out of your pocket after adding rent ) to claim back 40% of 40,850 = $ 16225 #
at 7.5% interest ($23,350 out of your pocket after adding rent ) to claim back 40% of 33,350 = $ 13340

Old
at 11.5% interest($28.700 out of your pocket after adding rent ) to claim back 40% of 28,700= $ 11480
at 10.5% interest($24,700 out of your pocket after adding rent ) to claim back 40% of 24,700= $ 9880
at 9.5% interest ($20,700 out of your pocket after adding rent ) to claim back 40% of 20,700 = $ 8280
at 8.5% interest ($16,700 out of your pocket after adding rent ) to claim back 40% of 16,700 = $ 6680
at 7.5% interest ($12,700 out of your pocket after adding rent ) to claim back 40% of 12,700 = $ 5080

Yes you can lock in a fixed rate but only for a certain time period and when that is over you could find the interest rate is 2% higher in four years time.

If you go down the line of joint ownership then half the loss and assign it to each taxable income.
However only $999 of the halved loss will be claimable at 30% for lower income worker
Any more loss will be at 15% for portion over $999 and below $28,000  of half of total loss amount

I can't advise you as I do not know your ownership structure, goals, financial position and risk adverseness
And I am not licensed to give financial advise.
 However I have pointed out the risk factor so you can decide the ownership structure (you need to talk to your accountant on what is the best ownership structure) and what risk you can tolerate.
I have not factored in annual repair costs in the calculations.