Storm in now or wait for the building finishes? Or, a third choice?

Hi all, very keen to get into the IP market at the moment. However I am faced with some choices, would love some outside advise from the more experienced than family and friends who did invest in IP about 5 years a ago.

Here is our situation: 

We are building a big house at the moment, colorbond roof is already up, in 2 weeks it will be in the lockup stages, therefore we should have the keys within 2-3 months from now. Saw a nice villa within 10kms of Perth GPO and the price is little bit lower than the market value. (I would say about $10000 – $15000) lower than the villas around it. Thing is we are thinking of making the offer and pay a large deposit for it so we can get the property while its cheap. But because of the negative gearing side, paying a large deposit is sometimes not the best choice when it comes to IP.

Should we:

1>wait till my own house is finished than wait for the chance to get other IP and let this one go so we can using the home equity to borrow as much as possible to invest and take the full advantages of the negative gearing bits. (because part of the income comes from small business, the bank will not lend us an additional $250000 on top of the money are already owe, even if we have made an additonal $100000 into our PPOR, the bank will still work on the original borrowing amount when it comes to buying an IP) <

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correct me if I am worng, been to a few banks and all the same answer.

2>We dive into this house while its cheap and pay a large deposit for it.

Here are our numbers:

expected rent for ip: $330 pw
deposit willing to put in: up to $90000
mortgage on ip: $250000
(we personally believe the IP will be neturally geared after fact in depreciations and building allowances, the villa is only 1 year old)
Current asking price is $330000 (I am confident the owner will sell for $320000)

my salary, 2 jobs + a very small business: $65000 pa
my wife: not working (studying full time)
our savings: $130000
our house value when it is fully built: $580000 – $600000
our mortgage amount: $324000
estimated equity: $280000

Interestingly, because of my wife not working, the banks I went to will only lend us an additional $90000 on top of our mortgage. Which leaves us the only option of going for a low doc loan, RAMS is currently offering us 6.89% on a interest free terms. Which is bit higher than the traditional loans. Should we go with them and pay the larger deposit for the IP? Or, is there a better option for us out there?

Many thanks