For spraying I'm wanting end up with a really smooth surface. All the
finishing work I've done before has been hand padding shellac. Is there
a gun that will give me the same results but faster?

The gun you choose will be largely dependant on the CFM capacity of your
compressor. Many HVLP spray guns need 10 or more CFM at 40psi, while some of
the newer gun designs can work with less, maybe 5-7 CFM. If you're going to
be finishing smaller items, a touch-up gun may be all you need. They have a
smaller spray pattern than regular guns, but also they use a lot less air
(2-3 CFM), making them useable with many of the smaller compressors.

Another factor you need to consider is the size of the nozzle that comes
with the gun. For spraying thinner coatings you need a smaller tip. The
better makes of spray guns will have a range of nozzle sizes available. If
you're not sure of what you'll need, the advice of a knowledgeable person is
invaluable. Someone else recommended homesteadfinishing.com and I second
that. Give them a call, tell them what finish you'll be spraying and the
capacity of your compressor and they'll recommend a model of gun and the
appropriate tip size.

If you're going to be doing a fair amount of finishing, that is, spraying
for a half hour or more at a time, you need to consider the duty cycle of
your compressor. Most portable compressors have a duty cycle of 50%, rated
per hour of usage. That is, no more than a half hour's use every hour. If
you run more than this over an extended period of time you'll likely damage
or burn out the compressor. If your spray gun needs 6 CFM you'll need 12 CFM
at the compressor for extended use.

If you're going to be spraying the odd box or two then perhaps a touch-up
gun is really all you need.