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
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.
375 USD
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
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.
There's no Glock with the model number "9", so I assume you mean a 9mm glock. The Glock 17 (full-size 9mm) holds 17 rounds in a factory magazine. The Glock 19 (compact 9mm) holds 15 rounds in a factory magazine. The Glock 26 (teeny-tiny 9mm) holds 10 rounds in a factory magazine. The Glock 17L/34 (competition) also holds 17 rounds. The Glock 18 (Select fire, full size 9mm) holds 33 rounds in a factory magazine. Higher capacity magazines fit and function in lower capacity firearms, for instance the model 26 can use model 17, 19, and 33 magazines.