I really think you need to discuss this with the shop you intend to use. They can educate you little on the process. I could give you general but there are veribales depending on exactly what paint system they use and they may not use a brand I sell or spray so I don't want to get too deep and speak for another brand of paint.

Thickness comes from color IE white is thicker than blacks or blues. Also how many mils. you apply. There is nothing wrong with base clear systems. I'd much rather have it than single stage. If the shop uses a quality clear and YOU take care of it, you paint job can last 20+ years. If they use a cheap clear and you don't take care of it yeah it's gone in 2-3 years. I'm not a big single stage fan it has its place but I'm not a fan for an exterior finish. Base clear systems have to be cleared or they will be flat and eaten by UV rays quickly.

Talk to your body shop and have them explain the exacts with you about the paint system they plan to spray. You could even leave and go do your homework after you know what they are going to spray.

Please don't take this the wrong way but I think you need more education on the subject and without knowing what they plan to spray I can't give it much attention. Auto paint is for sure something you get what you pay for.