1) I assume they don't have a tuner built in. How much do those cost and where do you get them? My Sony XBR tube unit has a separate tuner, but I suppose it's output is specific to the monitor
Depends on what you want to watch. I use a DTC-100 for OTA and Sat HDTV. You can get a tuner at any Circuit City, Best Buy etc.
2) Are line doublers neccesary? Is SVGA, or XVA (or whatever it's called) equivilent to 'hi-rez' viewing with DVD source? How much are doublers?
Most projs these days have excellent onboard scalers/line doublers, check here for more info on a specific proj
http://www.projectorcentral.com/ If you have a good prog scan DVD player, this is all you need for a great picture.
3) If the proj. is ceiling mounted, what do you do with the source units? Can the video cables be many feet long?
My panny AE-300 is ceiling mounted. Since the DTC-100 ouputs only vga, i have a 25ft vga cable that looks great for such a long distance. For Dvds, I use three RG6 Coax cables(cheap diy component cable) with RCA teminations, these also look great and are 25ft long. I have a 25ft s-video cable that I got on ebay for $7 that I use fo my PS2. I hardly ever use it, the pic quality isn't bad, but there is saying in the projector world: crap signal in looks even more crappy blown up on a big screen.
4) Are the $1200-$2000 projectors worth getting?
This is subjective, but if you have $1200-$2000 you will not believe the amazing picture you get for that price. Watching HD, esp sports on a 110" screen is awesome. My friends are amazed when I invite them over for the first time. This is where i did my research when I was looking to buy
http://www.avsforum.com/avs-vb/forumdisplay.php?s=5700f9f08da66c10c3c1c8aafde22003&forumid=68