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.
The Kel Tec in .40 is probably your best best.
Best to ask a gunsmith for help
yes
Very likely, it's the Glock. Glock in general is the best selling pistol in the commercial and law enforcement market.
40 USD or so
.40 cal hollow points. That is what they practice with also.
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.
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?"
It's very difficult to find a reliable .40 for under $400 in today's market, and considering the high pressure of the .40 cartridge, you don't want to buy a cheap .40 caliber handgun. If you really can't spend much money, the Kahr CW40 isn't a bad choice. I'm not sure how much they are at other places, but where I work, they are $450 or something very close to that.