Assume that you are going to need to do a little work on it: Most vintage bikes have been sitting around in storage for a while and will at least need to be regreased. You may also end up buying a new saddle, since fit is very specific to each person. Keep that in mind for your budget.

Newer Schwinns are made in China and are crap. 80s Schwinns were mostly made by Giant, and 70s lugged frames by Panasonic. These are good, as are U.S.-made Chicago Schwinns.

Figure out where you're going to ride: Boise itself is flat, which would make a single speed or 3-speed a good choice. Go into the mountains, and you'll want plenty of gears.

3-speeds are probably the best all-around commuter bike, although they're heavy. Sturmey Archer hubs will have the model and year stamped on the hub, i.e. AW 66 would be an AW hub (by far the most common) built in 1966. Older hubs are made better. These bikes will always have full fenders.

Surface rust on the rims is OK. Rust on the frame is not.

Also, remember that the proper number of bikes to own is n + 1, n being the number of bikes that you currently own. Don't worry if you get the absolute perfect bike for all situations, since you'll probably end up buying another bike after a while.