I also own a barco 808, so Let me give some input that I have discovered over the past few months...
1.) At 9000 hours, maybe? Depends how much effort your willing to put into the picture. I have spent many many hours tweaking to get the picture looking as good as I can. Since I got my projector for free, it was worth it all the way, and the image I get out of it is fantastic. If you don't want to put the time into it, then move on...
2.) I am also projecting onto a screen with ~3meter width. It is a bit dim, but if you have a nice light controlled room, and have a nice screen its not bad. The 808 is fairly bright. At that size, nothing is going to blind you without spending lots of money.
3.) Yeah, if you spend lots of time obsessing over the picture, focus it 10 times, adjust the astig 20 times, do the 3X5 card test, and repeat, you can actually get the barco 808 real nice and sharp. It still wont give a crystal clear 1080p due to the 8" tubes, but the 1080i looks fantastic, and has a much closer to film appearance than most digitals. (disclaimer: I am a movie theater projectionist, and have seen LOTS of film and digital) But, if you have the money, a 9" will definitely give you a sharper picture at higher bandwidth resolutions like 1080p.
4.) Does the picture look correctly color balanced? Human eyes percieve the colors at different intensity, so its hard to judge just by looking into the tubes. If the colors are off, look up how to adjust the G2 settings. There are some pots in the back of the projector that are simple to set if you take your time and are careful with the color balance.
5.) First, that is bad! There is a danger zone around the edge of the tube that you never want to shine onto (~1/4"). And, NEVER let it shine off the face of the tube. It looks like the corner is off of the face of the tube, and that is very bad. Make the image smaller, or shift it down so its centered on the raster. The curved sides are okay. When you finish setting it up, and converging the lenses and rasters, you will have different curves of your own. Unlike an LCD, the resolution is not fixed to a specific grid, and thus allows you to project a flat image onto a curved surface, and other fanciness. Being able to curve the sides allows you to adjust the keystone without loosing any video information unlike LCD projectors.
6.) If you have a card in port 3 (I did not) then you can make a special cable for vga to plug into it. Its cheap and easy to do. Then you can plug in your computer to it if you have a VGA connection. If your computer has a DVI-I port, then any adapter will work fine to power it. If you want to use port 5 (the 5 BNC cables) then you will have to get fancy on the software on your computer, or buy something that inverts the sync. (Google will turn up lots of discussion on sync inversion for Barco)
For CRT projectors, there is a WONDERFUL website with tutorials on how to do all of this, and with specific information about the Barco 808. www.curtpalme.com
Surf that site, and real ALL the setup guides a few times, then try to get a good install and convergence. Then read them again and redo the setup. Since you are a novice it will not be perfect the first few times, but it will get better the more times you do it, and the more time you spend doing it.