Mid 2007 20" iMac — Faint vertical lines on display, is this a serious problem?
I wouldn’t be concerned you could use the Mac for a week and then note for yourself if the situation is stable or if you want to start saving money to get it repaired.  We have machines like that that we use for timeclocks of people punching in/out - they’ve  been stable for years with graphics glitches similar to that. 

Keep toying around and figuring out how you can make the best use of this regift it would be my advice. I wouldn’t spend any money repairing it.  Save your budget so you could buy a new display if you actually need one.  Get one that would work with both the Mac and a low-cost computer like a raspberry pi.