Home theater setup recommendations?

My daughter, married, middle aged, has asked me to help them set up a decent home theater/listening room in their new home. I've been an audiophile since the 70's......before I even heard the term, so I'm a pretty competent 2 channel guy and I understand room acoustics............That said, I never really ventured into HT. I simply went with my 2 channel rig and was perfectly happy with things that way.....still am.

I guess my questions are..........How  important is multi-channel for movies vs audio.............dumb question, I'm sure, but it's not my area, so I'm asking...............and any recommendations for a decent, not UBER, sound system primarily for movies. They're willing to spend some money to get a nice setup, but not the kind of coin that many here likely would. I'm thinking used gear, speakers, subs, receiver or amps. Something that will sound good, but not require a second mortgage..................These folks think that hanging 4 inch Bose speakers in the corners of the room with a micro, wanna-be sub is good sound..............I want to "enlighten" them..........thanks