A DIY epoxy garage floor paint kit would save money and give you great results. Hybrid polymers, such as the one your contractor proposes to use, are blends of paint resins that bind well to cement and have very good UV and abrasion qualities. Applications are known as overlays and more commonly referred to as skim coats. Overlays are especially useful when the original concrete surface is worn, so using a hybrid polymer on a surface that's in good structural condition is overkill and very, very expensive.
I definitely think you can do this project yourself and save thousands of dollars. I'd recommend using an epoxy coating system such as QUIKRETE's Epoxy Garage Floor Coating Kit. Epoxy system kits typically include a cleaning/etching solution used to wash the old floor down and get it ready for the new finish, and a two-part epoxy mix that ensures a quick, thorough cure and superior adhesion. The kits also include color chips, those colorful flakes that are spread over the epoxy floor when wet to give it an attractive, textured finish when fully dry.
Epoxy delivers a coating twice as strong as concrete for the toughest garage floor in one coat and costs less than 30 cents a square foot, or about 20 times less than the hybrid polymer you were considering. I'd recommend you go with a DIY garage floor epoxy paint kit and use the money you save for some nice garage storage cabinets!