I agree with Grand Bois, if this is your goal:

"My basic thinking is to not strip the entire frame, but just smooth it down with 600 grit paper, and then re-spray several thin coats over the old (1970s) paint, and then wet sand and polish with polishing compound, apply new decals and wax."

But you started out talking about just touching up. For which I think an airbrush would be a great tool. But there's a strong learning curve using an airbrush.

The main problem I think you'll have, if over-spraying the entire frame, are all the nicks and dings. What you are now just touching up. If you go the complete over-spray route, and don't level those out first, you're going to see them no matter how many coats of paint you spray on. You've got to get the whole surface level first, then spray. Otherwise you're going to sanding every coat down to the original level of paint, until the divots are filled. A waste of effort, paint, and time.

Fill the chips and divots. Sand them down level with the original paint. It may take several fills. Then spray.

As far as airbrushes go, if you just want to try one for touch ups, I'd recommend as good a brush as you can afford. Like everything else, you get what you pay for. But good ones can be expensive and if you don't plan on doing a lot of this, it's hardly worth it. I own an Iwata, and it's a fantastic tool. But probably over-kill for what you're doing.

For full frame, it's hard to beat those detail guns. I own several conventional spray guns, including a detail gun, but if I were going to buy one now, I'd go for a LVLP (low volume low pressure) spray gun. Can be run on a relatively small compressor. Or, High Volume Low Pressure, if you have a compressor big enough to drive it. Which most homeowner compressors won't do. Not enough volume output. Beware of this difference if you are buying.