The Heckler & Koch P9S, a double action version of the rarer P9, has a large range of values in today's market. A well worn 9x19mm model, typically formerly used in police service, can fetch a price of $500-$600. Those in good condition with originally accessories typically sell for $800-$900.
The Heckler & Koch P9S semiautomatic pistol chambered in .45 ACP is quite a bit rarer than its 9x19mm counterpart, and can easily fetch $1,000 to $2,000 on the open market in good condition.
Can't be answered without the serial number.
Not possible to give you a single answer- depends on best for WHAT? The VP70Z was a full auto machine pistol, and the p9S a target pistol. Each a good gun for its purpose. The P7 series would be a good concealed carry gun, etc. Some folks like chocolate, some like strawberry- and the best ice cream or best pistol is a matter of opinion.
Depending on condition and exact version, anywhere from $2,000 to $4,000. You are going to need a "hands on" assessment.