Keep in mind - painting in your garage doesn't mean you're painting in your garage.
It means you' be painting the garage
If its not properly sealed up.
Not to mention, in order to keep dust down you'll want the door down. In which case by half way down one side you will no longer be able to see the car. Unlles you have adequate ventilation. You might get away with it on a full HVLP setup, but again I doubt you have a real HVLP gun, or a 13+CFM compressor to power it.
A booth is a tool, an extremely necessary one.
There's alot to a paint job. Wait that's not right at all..........there's alot to a properly done paint job - so don't rush it unless you don't mind if it shows.