I played these all when I was a kid. I wouldn't buy full price as many of the games dont run well on newer windows versions and modern graphics. However if there is a decent sale 60-90% off I would recommend.
KQ 1-4 are old school VGA with text interface. I would recommend you just download and play the AGDI remakes (which are free and prettier, none for KQ4).
KQ5 is the first with the pretty 90s 256-VGA graphics, although this version has an annoying voice acted script without text. This is frustrating because when you want to move quickly from one interaction to the next it is faster to read a textbox then listen to the lame voice acting. Even modern games like Skyrim etc. appreciate this and always have a text option.
KQ6 is great, the best of the series IMO. Excellent plot with fun characters and a challenging set of puzzles, nice 256 VGA graphics, text box.
KQ7 is lame, the classic VGA graphics are replaced with disney-esque cartoons and very buggy. You need to go online to find workarounds for a couple of the puzzles that don't work right.
Overall, very nostalgic and enjoyable to play, but wouldnt pay more than 5$ for because of compatability issues.