If you work as a contractor, definitely you will need an ABN. The application for it is easy and may take up to 4 weeks, if I remember correctly.
I don't remember ABN necessarily has direct impact on your tax claim. I have my ABN for 6 years and has not used it that much. It did not cause much trouble with my tax claim either.
You can keep the ABN as long as you like and I don't see any negative impact from keeping it, as far as I know.
To sum up, if you are going to work as a contractor along the road, go for it. It does not have negative impact so far as I know.
you can apply an ABN online if you have a tax file number.
If you apply a sole trader, the ABN is connected with your TFN. As the above mentioned, it does not cause much trouble for you to report your tax even you have a permanent job. you can have both of them.
Yes, you can claim the tax back if you use that for your work, you also can calim part of the cost of your laptop as expense for you to earn the income, depaends on how many percentage you use your computer to do your work, just be reasonable. (as long as you have the tax invoice when you bought it with you ). so it's good for your to have an ABN, you even can calim the transport cost as the expense.