Interest Only loans. Do i ask for 1 year, 5 years or 7 years ?????? with the remainder Int+princ repayments

Hi Everyone, please educate and help me.

I am currently in the process of buying a property which has a lease of 7 more years on in (end 2017). I've just got conditional approval from my broker but they said it is currently set to 5-years interest free (westpacs flex first investment loan). I asked if i could change it something lower and they said they could ask. As this is a department of defence housing i have guaranteed income for 7 years, i need to pay a hefty 13% x rent "admin-like fee" each week.

I just want to pay off this home loan as quick as possible but i don't know how significant this interest only period things will be? I know theres an element of saving tax (is there a good web tool to estimate this) and having to only pay the interest fress up some cash at the start of the mortgage .. which i could use to earn a greater return .. etc..etc .. but i doubt i would put spare cash to good use. Leaving it in a ING account is prob worse off than just paying of the mortgage right?

My Questions:
[1] Do i try get it as close to a pure interest+principle repayment like loan
[2] If i stick with the 5 year (or maybe greater) interest only loan … what do i .. or what do people who are on these types of loans do with their cash and taxes during that period.
[3] Is there any good interest only for xx years then interest + principle loan calculators on the net? Ones that show balance each year and if i made a lump sum deposit after the interest on period?

Many thanks, i know this type of scenario has probably been asked before … i'm going through the threads. .. i'm just hoping someone can help fast track my learning as i need to make the decision soon.

Greatly appreciated ,

G