your on the right track.... its called reducer. Typically your basecoat (color) is mixed 50/50 with reducer, so 2 quarts of pure basecoat once reduced yields one sprayable gallon of paint. Clear coat is reduced differently depending on the brand. its usually between 10-30% ratio. The more you reduce it, the more it "levels out" yielding a smoother surface finish and less orange peel, on the flip side, its also more prone to have runs, which are a PITA to get out.
When we sprayed my car, we put down 3 coats of clear reduced 15%, the orange peel was a little more than I had wanted, so the 4th and final coat we reduced it 20% and I had a few runs on the bumper and edge of the door. It was a PITA to get it out, but they are out, it just involves very very careful wetsanding and good polishing.
That price of $6k isnt bad because that includes a new OEM 1/4 panel and the install of the 1/4 panel, typical 1/4 panel install labor is roughly 14-16 hours, plus the part is about $400 or so (the shop is charging you LIST price, not ebay or hookup prices). So figure thats about $1500 worth of work when its said and done, so the other body work and respray is $4500, which isnt too bad. The front and rear glass have to come out of the car either way for a good paint job, the idea is not to leave any "open breaks" in the refinish. An open break is when you stop the paint/clear coat mid panel, or near the edge but not to the edge. On a roof for example you want to pull the front and rear glass out so you can end the refinish under the glass seals, this will prevent the clear from peeling or flaking off from moisture or debris getting under the edge.