mxz-man

Hi all, I'm new to the forum and have a few issues with my 2000cavalier that id like to get some insight on.

Background. Bought the car for a temporary ride. (2.2L 5spd) When I test drove it started and ran good. 3 days later I go to pick it up and on the way home it starts thumping like the engine is seizing. Shut it off immediately and pull over and check the oil. Its very low so I fill it up an try my luck on my way home again with no problems. So i know the car has somewhat seized the engine but i don't know how bad it was. After that i drove the car 3 hours straight on the hwy around 120-130kmhr. with no issues. A few days later I go to drive again (not as bad as before) and now I'm randomly getting thumps like its doing it again but only at lower RPMs and usually decelerating in second to fourth gear. Like i said I had zero problems on the hwy at 130.
Anybody have any idea? I'm guessing I've got at least one or two pistons /cylinders that are shot but i dont know and wonder how much further the car will go before it goes kaboom.

Also having issues with an odd hanging idle sometimes.. my guess is leaking vaccume lines but im no mechanic.