@nura, more or less the same issues I had in my mind when I was thinking of changing my Palio.
The installation challenge will be the 3-way speakers (specifically the midrange) and not the HU or amp or anything else.
IMO the best car (with a boot) to ICE is a Verna. Great metal, great plastics, minimum damping. Rather than a Santro, an i10 would be a better choice. So would be a Getz.
In either case, the Midrange will be a bit difficult to install, but it can be done (without cutting the door), if you get a small FG pod made, or if you get a pod-like spacer made from MDF (paste & screw multiple spacer rings together and cut around them to shape). The pod can be screwed to the door. The midrange is pretty small and shallow, so we will not need to cut the door pad.
Making custom door-pads for the newer cars will be tough, without major FG work, as the door-pads are not straight, and are moulded to shapes in plastic.
Now coming back to the car, I settled for a Swift for a whole lot of reasons not totally ICE related. But I did consult extensively with my installer. The Swift has a pretty deep dashboard with quite a bit of flat area. And the corner between the dash, the windscreen and the A-pillar gives quite a bit of possibilities for housing the midrange and tweeter in a pod, without affecting visibility too much. The downside is that actually getting a pod done there might cause the mids and tweets to fire a bit into the windshield (if mounted flat, firing up/angled up), causing smear. But my installer and I both think that this can be avoided to a large extent.
In my case the midrange is 4" (Energy Series). But you have it a bit easier, because your midrange is 3", and a bit shallower (Hi-Energy series). So lesser issues.
You can also go for a door-pad install, with pods for the midranges fixed there (like LBM's current setup).
I will be getting my ICE reinstalled in the Swift in about 2-3 month's time, so that might be of help (if you decide to go for a DZire -- A DZire and Swift are identical inside). I doubt if you'll get delivery by then anyway
The DZire will take 6.5" midbasses in the doors, no issues there.
Be warned that a DZire will need a looooooooot of damping, at least double that of other cars. I'm looking at about 15K as a damping budget for the Swift. I'll also get underbody coating from 3M or Dinitrol done, and that will help with sound-deadening (lessening road-noise) a little bit.
Another option is the Fiesta -- the stock pods are high up on the door panel, and take 5x7 speakers. Perfect position for a midrange & tweeter. A custom spacer can be made (in 5x7 sixe) which will take your 3" midrange and a tweeter,
and enclose them both inside the stock grille. But installing the midbass will be an issue -- the door will not take a midbass in the usual position (lower forward corner) without a gigantic spacer (if at all it can be done). Maybe, just maybe, the midbasses can be installed under the seats in small boxes, but well, how the whole setup will sound when it all comes together, is something we can't predict in advance.
I'm not sure if the Ikon has the same door-pad setup as the Fiesta. The old ones (up to 3 years older) did not.
As DerAlte said, the TATA sedans could be the perfect answer, if you're OK with TATA, and if they are like the Indica. If they are (like the Indica), you get a Midbass pod in the door, and a 5.25" pod in the dash. Perfect. The tweeter can be installed close by to the midrange (which will be in the 5.25" pod).
All the best in choosing the car
EDIT: Dash kits for the Swift ZXI and the DZires with the integrated HU are easily available now. My installer had done quite a few HU replacements in both over the last few weeks.