Rattle cans are cheap & effective for small projects, if you try to paint an entire car with them, the results are not going to be an "automotive quality" paint job - and this is coming from someone with an entire shelf full of half empty rattle cans in varying colors, and who has used rattle cans for automotive trim & touch up.
I can't believe that someone used the term "decent gun" in the same sentence as Harbor Freight & Northern Tool - don't get me wrong, I love going in there, and browsing, and occasionally buying something - that would be a cheap gun, and you will get cheap results, but, by the time you've invested in a gun, compressor et all, it won't be a cheap paint job, especially if you have limited experience.
I do agree that prep work is what makes or breaks the job.
93 - 1.3 Suzuki Swift GLX
98 - 1.8 Mitsubishi Pajero iO
98 - 2.0 Suzuki Grand Vitara