A few things from 45 years in the business...and my opinions
Pretty much any paint these days is OK, from enamels to urathane basecoats. It is true that Duplicolor lacquer has plenty of advantages for do-it-yourselfers...dries fastest, easy to spray, premixed, not expensive, no special booth or safety equipment. You just pick whats best for the way your going to use the car. Lacquer will REQUIRE buffing to have any gloss and takes a lot more upkeep. For your run of the mill daily driver paint on a budget, I recommend acrylic enamel. Fair gloss, easy upkeep, cheap, no clearcoat needed. Primer is your choice...pros and cons for both kinds (lacquer and catalyzed 2 part). All prep is pretty much the same no matter what you use.
Flex agent is for soft interior parts like padded dashes and door panels. Invented for painting original rubber 60s GTO/ Firebird bumpers. Didnt work then, does absolutely nothing now. It works fine if your painting a rubber soccer ball. Contrary to Jimmo, IF you use flex...it MUST be in all coats. In fact even a flexible primer is recommended. Basecoat/clearcoat finishes are flexible on their own....enough that they can bend like a piece of paper. The flexible bumpers on my Sebring have been painted going on 6 years with barely any chips, and no cracking or peeling anywhere. Its my daily all weather driver AND my current show car.
80s basecoat/clearcoats were far different than todays. Originally the color base coats were practically lacquer, and the clear was not a true urathane. They seldom stayed on the car for more than a few years. Either basecoat peeled off the primer, or clear seperated from the base color. Best to remove at least the original clear unless its still in good shape. Sand it, DONT use any chemical strippers (or do so at your own risk). I can strip most of the paint off of just about any car sanding it with a DA in a day or two. Stripper can open a whole new can of worms.
House of Kolor has awesome colors and paint is good, but expensive (overpriced). Their clears are trash.
[This message has been edited by rogergarrison (edited 09-24-2013).]