First Q: Yes, it would looks as crappy as it does when you push your computer monitor to anything less then native.
Long cable is fine. Obviously we're not talking about 1000' here, are we?
HDMI should go directly into the Tv, nothing wrong with that
As long as he builds a rig with a decent video card like 5770 (minimum) then he'll be fine. And to be quite honest it must be nice to use a 46" tv as a monitor. I tried my 32" hdtv once and it looks pretty good, just not right up to your face.
Just plug in the monitor as second display and the 19" as main. Then just have windows copy the desktop to both.