Do you still think there is better stuff there in the same price range?

Yup.

https://www.amazon.com/BS5-Passive-Bookshelf-Speakers-Tweeter/dp/B07YZDS9LF

https://www.bhphotovideo.com/c/product/1628457-REG/kanto_living_yup6_yu_passive_5_25_bookshelf.html

Those are $110 and $180, respectively. Both are much better than the Klipsch.

Also, the R-120SW is currently discounted to $299 on Amazon so you'd have to be nuts to pay $310 used.

I wouldn't recommend it for $299 either BTW. Get the BIC F12 instead. Better sub for the same price.

https://www.amazon.com/BIC-America-F12-475-Watt-Subwoofer/dp/B0015A8Y5M

Different amp/mixer wouldn't really change much.

The JBL sub you linked would probably work just make sure you don't pump too much power into it; it may even have built in shut-off if it feels threatened.

You could probably get away with these, also; BIC America F12, Theater Solutions SUB12D

Though I wouldn't usually recommend using 'home audio' equipment for uses like yours, for your price, it seems the only way to go. Just make sure you don't push the sub too hard and keep and eye on it and it should be fine.