I have made makeshift paint booths myself a lot of times. My makeshift booth was usually a shamiyaana with additional plastic and a mean machine squirrel cage blower to exhaust it. And 3 of 500W light fixtures with reflectors.
a photography fan freind of mine taught me how to work with reflections and such to see results.
btw - this may make people look at you like you have horns and a tail. The usual case was that all hired help would step aside and just look at me doing work instead of being the "hired help" - kaam chor, but you cant have your cake and eat it too.
for metalwork - if the original metal is really bad - I would suggest to either find an expert panel beater who can cold roll new sheet metal into shape and smooth it on an english wheel or buy new assemblies.
The smoothest paint finish is 99.95% dependant on the preparation. It takes a lot of swearing, sweating, odd hours, lighting, carpentry and even some automotive voodoo to make a side panel of a car shine like one piece of mirror. Its tough but possible.