Mukesh, I'll try and answer the questions as best I can. Some of what you have asked is subjective matters, so my answers hold no empirical sanctity.
Is the Hawk worth the price? You have to decide that for yourself. I consider it overpriced.
How much testing will a dealer do before handing over the vehicle to you? I would say nothing more than about 20 kms! They do not have to take the car out on the roads for 'testing'. They simply have to do a PDI as per a checklist that you can ask to see.
As for the stereo system, keep the original head unit in place so that you retain the steering wheel mounted controls. For the rear channels you could deploy a separate amp to power the speakers and the sub. Since the amp will be fed from the HU in the dash, you'd be setting up the gain of the amp by using the fader control of the HU.
BTW, in all fairness, if you are looking simply for a more comfortable ride than a Zen, my choice would run to the Innova as being technically the best.
The Safari is supposed to have the best back seat comfort.
If you cannot accept the looks of the Innova, well, tough luck. You're pasing up the best vehicle in that budget that India has to offer.
I went in for a Scorpio because I wanted a 4WD vehicle. I opted for the Scorpio for the simple reason that it is more reliable than a Safari, or so I believe. I am a satisfied owner upto now (little over 2 years and 16000 kms). No petty/niggling issues and no major issues.
About the coatings for the car, I'd say it is a debatable issue. What is recommended is an underbody protection coat, but even this can wait for a year or two. The new car does not really need any of these expensive coatings that dealers always try to sell to you.
Do take the extended warranty. M&M offer 2 additional years for Rs. 10,000/-, but that was for the 2.6 CRDe.
One important point to bear in mind: You can save yourself a bundle of money by getting your No-Claim-Bonus from the Insurance company transferred to the new car. You'd have to then transfer the ownership of the Zen to someone else's name and buy a fresh insurance for your Zen.
Just remember that the NCB accrues to a person, not a vehicle.