The lowest end bodies have lower QC, and you are more like to end up with random issues that may affect picture quality: unwanted film curvature, shutter speed not so accurate (affects exposure), etc. i believe the middle-range cameras ($400-$800?) should be a safer bet than the absolute cheapest bodies.
Other small things can indirectly affect picture quality: in full manual, a 100% viewfinder helps, as well as having a real prism instead of a mirror-prism; for landscapes, having mirror lockup helps when the mirror damping mechanism is shoddy. (Won't mention AF, since you already specified manual focus.)
Oh yes, the metering: i've used a midrange (EOS 50) body and a higher end (EOS 3) body before - the higher end body has much better metering in a wider variety of situations, esp backlit photos. Of course, if you meter manually/externally, then it doesn't matter. The EOS 3 also has spot metering, which gives more control for manual metering.