Hi Zareline
Roads in Aust are v good. Dun worry.
I only caution against exhaustion and road kill.
Make sure you have enough rest and have frequent breaks when you're on the road. Driving continuously is v dangerous. Besides, Tas is so picturesque that u will often want to stop the car to look around.
Booking a car is v easy. Book online at the car rental company's aussie website. eg Budget, Hertz, Delta etc they will have offers regularly. Tho i must inform u first that car rental in Tas is slightly higher than in mainland Aus coz Tas is an island on its own, everything they lack has to be "imported" from mainland.
I personally recommend Hawk rent a car, based in Singapore. They're v affordable, even cheaper than the previous times i used Budget & Hertz. Check their website at:
www.hawkrentacar.com
Mar is late summer. Better to bring a medium-thick sweater along. Tas is generally colder than the coastline cities of Aus (eg Melb) coz the wind is blowing in from over the artics. so its colder... regardless whether its summer or not.
I recommend you get a sweater that water-proof outside also, just in case it rains. Showers are quite common in Tas. When i was there, i witness Hail when in was in launceston. Lasted only about 5 mins, but definitely an experience!
my 10 days cost me about $4k+. There's really nothing much to shop, esp since the shops close early. The fashion in Tas is also more expensivve and alittle behind time compared to Melbourne (its next nearest Aus city). Next to note is that my accomodation varies from $60+ per night to $150 per night, depending on where i stay. Ranged from the hotel rooms above a bar (yucks!) , to motels (my usual), to 4* Heritage Bed & Breakfast, to 5* Cradle Mountain Lodge etc. It really depends on you.