to answer your questions, 1. You should use the same brand of thinner and hardener as the paint. Not just brand (Dupont, PPG, R-M, etc) but also for the same paint type. Urethane, Acrylic Enamel, etc...
2. Reducer for the most part is another name for thinner. Hardener is not required but recommended for acrylic enamel, required for most urethanes though. It helps the paint dry (chemical reaction) as well as helps it cure harder.
3. Acrylic enamel is not bad if you are use to painting it, anything can be difficult if you are use to something different!
Check with your painter, some painters are use to a certain brand and type of paint, not all are created equal!
