It all depends on you, your pocketbook, your skills.
No.
$489!!
45 has the larger projectile. Size of the handgun will vary.
Yes
Either platform can be larger than the other. The 45 has the larger projectile. Caliber is defined as the diameter in fractions of an inch, so .40 caliber is smaller in diameter than .45 caliber. Caliber can also be defined in the metric system, measured in milimeters. The .40 caliber cartridge is equivalent to 10mm in diameter. As noted, the size of a handgun can vary widely within one caliber, so it is perfectly reasonable to have a smaller handgun crafted in a larger caliber.
it is a little less powerful but not by much
100-500 USD depending on EXACTLY what 40 caliber mode you have.
A 40mm handgun would fire a bullet the size of a golf ball. No such critter. There IS a .40 caliber handgun (about 10mm). There are dozens of larger caliber handguns than .40 caliber. They include the .41, .44. .45, .455. .460, .475, .500- and several others.
The model 19 is a 9mm only proposition. The model 19 is not available in .40 caliber, but the model 23 is the same size gun and it is a .40 caliber gun. Some people would say that the model 23 is the .40 caliber version of the model 19.
50-500 usd
No. You can only safely fire the correct caliber from each handgun. For instance, for a .40 caliber handgun, you can only fire .40 ammunition. There are some minor exceptions, mostly in revolvers. For instance, you can fire .38 special or .357 magnum from a revolver that will fire .357 magnum, but not the other way around.
It depends on several factors, but a good rule of thumb is "about a mile."