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As modena mentioned, the cfm required to run an air tool like a sander is relatively high. Look at the specs for the tools. Unless you are going to get "into" this work, air tools are a whole parallel world. what you will spend getting into it will buy more than one electric sander.
Electric sanders are good, buy what you can, use air to clean it out and keep sanding.
Velcro backed sanding pads are a blessing. Look for larger packages (<$) An auto paint supplier may be a good place to start.
If you can hook up a vacuum to the sander it will take care of most of the dust at the source. (duck tape will help).
Rough enough grit to knock off the bumps, and fine enough to keep from ruining the project.
Good Luck!