I just got a Sony 55" x700d and love it. For a cheap TV it's great. You really can't go wrong with a Vizio, but be aware that their warranty support won't be what one of the top tier brands offer. However, you can't go wrong with how good the PQ is with Vizio.
As for low end brands using cheap panels, well yes and no. Normally they use b-stock or older panels, but that is completely brand dependent. Vizio is trying to be a top tier brand so they use high quality parts.
As far as going 1080p vs 4k I can agree that there isn't much of a point. I wanted to play around with HDR more than anything and it's pretty sweet. I will say that depending on where you look 4k and 1080p tvs can be very close in price.