La Mer,

Slow down. As soon as you report in to the Qatar Airways office for on your first day they will take away your passport and you will not get to see it for a month cuz they will try to get you a work visa. They will then give you a photo copy of your passport with the entry stamp page to doha. That is that copy that you can use to get your SIM card with QTEL. When you get to QTEL make sure that you tell them to make a new copy of your Photo copy passport cuz you will not be able to ask for a new copy with HR when you actually used them all up.

As for QTEL location you can look it up on the following link or just tell the taxi to take you to Lu Lu Hyper Market (QTEL on second Floor) or City Center Shopping Center (QTEL on second floor also I think)
http://www.qtel.com.qa/PrepaidHala.do?prodtype=1 Look at this link for more details about Halla prepaid SIM card

For Qatari driver license, you do need to have your work visa to get one. Unless you are willing to get a temporary driver license which only valid for a few months but it is the same price as a five years license so I suggest you wait for your RP (Work visa) As a cabin crew you will get free transport to and from work (Official business) so I think you should wait a bit longer on buying car. And yes, you do need to have RP before you can finance anything unless you are willing to pay cash. If you still want to drive then you must bring along your International Driver License from your home country. With that license you are allow to drive in the country(Qatar) for 6 months, but most rental car company will only rent a car to you for 1-2 months maximum with INTL Driver license. It's all varies with each rental car company policy. For a small compact car rental rate you can pay from 1,600 - 2,000+ Qatari Riyals per month. (Nissan Tida, Honda City)

Hope that these information are some what helpful to you and goodluck with QR. You join at the right time cuz they will increase your salary next year. I'm not going to tell you how much, but everyone is happy so far.