How long also depends on tank size. I think the most common tanks are 5 pound and 20 pound, but they have other sizes too. A 5 pound won't last nearly as long as a 20 pound, but a 20 pound CO2 tank is a pretty big boy...heavy too. So when you decide what to get, figure in the trouble you'd have storing it and taking it for refills.
Stick with CO2. Nitrogen tanks are filled to a much higher pressure (something like 3000 psi), so if you knock it over and break off the neck, the tank is going through your wall, your car, your shed, your neighbors house, etc.
I, literally less than a year ago, sold a '55 Coronet that was a virtual twin to the one in your profile pic, except mine was the reverse colors (blue with a white hood)...beautiful car, but couldn't keep it because of family situations.
I actually use the grit on homemade tools mostly for cleaning up metal gaming minis. As for plastic, it works well for small detail parts and engines and whatnot, since you can make small tools in different shapes.
I also make my own padded sanding sticks. I use what they call eva foam...comes from the craft store or the craft section at walmart in sheets...I glue down a whole sheet to the back of a sheet of sandpaper, then to the back of the foam I glue down a bunch of popsicle sticks or bits of wood...let dry and cut up. They don't last as long as the pre-made kind from the hobby shop, but they are a lot cheaper, and I can mass produce them.
That famous auction site...search for tumbler grit. I got a little bag of 400 grit that has lasted me for years with no end in site. My favorite is to put glue on the ends of toothpicks and stick them in the container and let them dry, but you can use any shape you want.
I'm really getting a good chuckle over the cat claws in this thread. Ya'll fight like a bunch of wet hens.
For what it's worth, my two cents is that if you spray flammable materials through a fan that isn't designed for it because you're too cheap to buy the correct kind of fan, you deserve to watch your house burn down...and just because you've been doing it for xx years with no problems doesn't mean something bad won't happen tomorrow. I once pulled the ignition coil wire off a running engine with my bare hand and found out just how far 30,000 volts can jump...just because I'm still alive doesn't mean I should keep doing it...it just means I was luckier than I was stupid.
Having said that, I have a homemade spray booth (with a proper squirrel cage blower), but I need more CFM. Has anyone seen the fan below? It seems to be sort of a squirrel cage type blower, and looks to be easy to modify for a custom spray booth. If it's "explosion proof" and pulls anywhere near the rated CFM, it would seem to be the best buy around, but I can't find anything specific about the motor on the company's website.
Just make sure you carefully prep your parts...I sent some stuff off to them and they came back perfect...so perfect in fact that they showed off every single little gouge or rough spot I left on the parts like they were under a spotlight. Nothing will make you realize what a crappy job you did of prepping a part like a nice new coat of chrome. Taught me a lesson about putting some elbow grease behind the 1000 grit.