1. What voltages do sonars like that usually work? Is it okay if I give it 6V, or do I need to add a 5V voltage regulator for it?
Could you help me powering all the components? I don't really know how much current can the arduino give :-?
Presuming the ultrasonic sensor is the HC-SR04, the datasheet says it uses 15mA at 5V. Since the Arduino is able to power 20mA per pin, and that has a regulated 5V supply so that sensor will be able to be powered and run directly from the Arduino.
2. Should I use the L293D integrated motor driver or use the H bridge from the right? (I just have no idea what transistors to buy for it..)
That's entirely your call, but if you can stretch the budget to something prebuilt that will remove one potential set of headaches from your future. Once again you'll need to consult the datasheet for the L293D to ensure that it has the capability to supply enough power to control the motors you're using. There are beefier alternatives out there if not.
3. Someone told me that I should use Eagle instead of Fritzing - for PCB layout.. does it worth the switch..?
That depends... Fritzing is easy to use, and has a PCB manufacturing service at the end of the process and if you're looking to produce quick images of a breadboard with your circuit laid out on it, you get that too. Eagle makes great looking schematics, has loads of parts available in libraries available from a bunch of sources online, and can create the gerber files for sending to a wide range of PCB fabricators. It does have a steeper learning curve, but it's a far more capable design product. It has a huge ecosystem of plug-ins to expand its capabilities too. Depends if you need to add another big learning curve to the one you're already due to face with this project in my view. If you're wanting to use a PCB fabricator other than the Fritzing guys, I am not certain their software can do that... At the end of the day there's nothing to stop you using both - the price is right after all :)
Hope that helps,