Very likely, it's the Glock. Glock in general is the best selling pistol in the commercial and law enforcement market.
The Kel Tec in .40 is probably your best best.
Best to ask a gunsmith for help
yes
.40 cal hollow points. That is what they practice with also.
40 USD or so
That's really too broad of a question to answer accurately. Best for what? If you happen to mean best quality, Sig, Glock, and H&K are some of the best.
5-250 USD or so
If you go to S&W's website you can get a manual for free.
Depends on model and condition. Sort of like "How much is a Ford car?"
No one answer- usually 2-3 letters, and they vary depending on the model.
As far as taking it apart, it depends on the model. Check your user manual. If you don't have one, call the manufacturer.
It all depends on you, your pocketbook, your skills.