You could expect to pay between $450 - $550 for a basic model with basic accessories in good condition.
100-500
brand new roughly $450
I just bought one for $499.00.
Yes
375 USD
No, they are different calibers. A Glock 17 is 9mm and a Glock 23 is .40 S&W. A Glock 22 magazine would fit in a Glock 23.
Glock never made a model 8. You may be thinking of the Glock 18, their only full-auto handgun, which was developed from the 17 for Austria's "Cobra" counter-terrorism unit.
You cannot just swamp them out no.
Glock 9MM Model's 17/19
The number of bullets a Glock can hold depends on the specific model and magazine used. For example, a standard Glock 17 has a magazine capacity of 17 rounds, while a Glock 19 typically holds 15 rounds. There are also extended magazines available that can increase capacity. Always check local laws and regulations regarding magazine capacity.
100-400 USD or so depending on specifics
Glock 18 was first introduced in 1985. It is made by Austria's Glock AG, it comes in two variants - basic Model 18 and Model 18C. It can have a 10, 17, 19 or a 33 round magazine.