Frankly if I were down to choosing just one handgun, I would approach it this way.
I would get a SIG 226 in .40 S&W and then purchase a factory .357 SIG barrel. This will give you two magazines that can be used in both .40 and the .357 SIG
Then I would purchase an after-market 9mm barrel and two magazines.
SIG also sells a .22 slide and mag combo for the gun.
image from SIG's website
This would basically cover several self defense calibers that one might need plus allow you to use .22 LR for plinking, using the same grip and trigger system (in a configuration with guaranteed performance).
Kinda like the Swiss Army knife of handguns, you can do this on several SIG models (like the P229.)
However due to PB constraints and ammo availability, this might not be such a good idea for India.
If I was in India, being subject to Indian rules, I would consider the Ruger Blackhawk in .327 Federal Magnum. This will allow the firing of .32 S&W and .32 S&W long in addition of .32 Federal Mag.
image from Ruger's website
Ruger also made a Single Six in .32 H&R Mag at one time (See
Here) which would be smaller than a Blackhawk.
image fromHere
Ruger also makes a 3 inch barreled Ruger SP101 in .327 Federal Mag.
image from Ruger's website
Winnie,
I'm sure you are aware that those HK convertibles are collectors items now and probably cost an arm and a leg and your first born. They also might be very hard to find. The concept was nice though.