If you want to be independent from your son's viewing the Slingbox will need it's own dedicated TV source, i.e. another cable box. How you do that we can't know, as it depends on your Comcast setup. All the Slingbox does is take a TV signal and 'sling' it onto the Internet.
There is no 'best' Slingbox to buy, it depends on what you want. Just go to the Sling site and check them out.
There is no requirement for a TV in California, apart from the one your son is using. The quality will depend on your Upload speed at CA. If it is 2Mb+ then you should be fine, although I actually get a 'watchable' picture (i.e. I can follow the plot but not read any text) with just a 0.4Mb upload speed.