I was thinking about doing the same thing...how hard is it to do yourself? I have never installed stuff in a car before but I am good with installing and hooking up different kinds of electronics. Where would the TV Tuner go? And the DVD Player?
Both TV Tuner and DVD would go into the video input on the in dash monitor. There are some units out that that have TV Tuner and DVD built in. Just search around you'll find a perfect Mobile Video System.
It's not hard to DIY if you know what you're doing. I installed my indash DVD-monitor myself with no problems...but, I've done installs for a very long time now.
As far as building your own base, just get a dash kit. Unless you plan on putting in 2 single DIN's, them you probably have to build it yourself. If it's a single DIN, then buy a dash kit for your alty. They do make one for the 3rd gen. Search, or ask your local stereo dealer for one. It costs about $10-$15.
Just follow instructions very carefully. If you run into the 'brake' safety wire, ground it to bypass it.