10mm is actually .40" caliber rounded up...
The .40 S&W and 10mm are the same diameter, BUT, 10mm is a longer cartridge with a heavier bullet and more powder...
Since they are the same diameter but different lengths, .40 S&W rounds will actually fit in a 10mm pistol, and they will actually fire, there is a debate on how safe it is to shoot .40 S&W out of a 10mm pistol... Obviously you won't be able to fit the longer 10mm round in a .40 S&W pistol...
The 10mm round has almost as much mass as the .45ACP and just short of the muzzle velocity of 9mm, so it has even more kick than .45 ACP.... ...I'm not to familiar with .40 S&W, I think its about the same as 9mm, I might be wrong...
10mm having even more kick than the .45 ACP, I think you can find more than one person that would suggest 10mm is not the first pistol to start out with.... ....at the same time, the first pistol I ever shot was a .45 ACP.... ....so its not like you can't learn with a bigger caliber... ...ideally it is easier starting with a smaller caliber and working your way up, but IMO, if you're only getting one pistol for a while, get the one you want, you can learn to shoot it, even if its a bigger caliber...
And yes, if you're lefty, you want an Ambidextrous Pistol. Sure, a lefty can fire a pistol designed for a righty, but they have to adapt to it and do things differently than the right hander, like using different fingers for the Magazine Release and use the other hand to release the slide when its locked back.
Since this is going to be "your" pistol, I would get a full ambidextrous one, one that has safety, magazine release and slide catches on both sides of the pistol...
The SA XD-M Elites are fully ambidextrous and come in 10mm and I think .40 S&W also...