Alot of work, alot of money, and a long time to make it profitable.
You will never make a living in a small shop like you could if you just worked for someone else. Unless you have the work and the drive to manage more then 4 guys you might as well stay small. But unless your John Buterra talented, people will never wait the million years it would take to finish a noteworthy car. You have to make periodic splashes in the publicity game to keep peoples interest. Which takes manpower that is very difficult to find unless you just want to hack up cars. If you want to hack up cars your business will last long enough to burn every possible bridge in a 360 degree circle around your shop. But hacker employees are easy to come by. But if you do quality work don't be enticed to think you have to price yourself to compete with the hacksters.
OH and The worst customers are the ones I gave the biggest discounts too.
Most of these lessons took me years to learn, I will let you know on the profitability thing.
Oh and the best way to ruin a great hobby is to try and make a living at it.
But I would not trade building cars for any other career. Well maybe a few careers........