I will relate my experiences of what I did in Sumatra in Jan/Feb 2005.
I hired a car for 150,000/day and a driver for 60,000/day from my friend Eddy in Painan( 2hrs south of Padang) see photographs and travelled through West Sumatra for 19 days. The price for the car was a rock bottom price and I would think you could expect to pay about 200,000 from Eddy now.
I paid for all the petrol, and food and accomodation for the driver.
We travelled from Painan to Padang, to Pariamin, to Embun Pagi, Lake Maninjau, Bukittinggi, Payakumbuh, Solok, Muara Labuh, Kersik Tao, Kerinci, Sungai Penuh, Muara Sako, Tapan and back to Painan. Took 19 days and was an absolutely fabulous trip. Had seen most of the places before, but it was fantastic seeing them all again plus new ones like Muara Sako and Lake Maninjau, both just heavenly.
Here is my photos taken on the tripcommunity.webshots.com/user/ppamela1
I flew from Padang to Medan and hired a car/driver whom I met in the shop where I was buying a phone card for my mobile. I was not intending to hire a car, but was not feeling well and believed I would be better, feeling sick in Bukit Lawang than polluted old Medan.
It cost me 300,000 rupiah for the one way trip, quite expensive, but it was a very nice modern air con van, though I did not use the air con because I cannot stand it.
Sorry I do not know how much it would cost for a driver and a car from Medan.
Personally I think it would be better to hire a car just for Sumatra and then engage another car and driver whilst in Java.
Have contacts in Bogor Java who might be able to help with this.
How long will you be in Indonesia?
I am headed back for 2-4 months from June 21st flying into Bali and travelling from Lombok and maybe Sumbawa right through West, Central and East Java, West Sumatra, North Sumatra and to Pulau Weh and Banda Aceh.
You are more than welcome to email me at pamelajoyfuge