There are some who do a decent job but not a professional one by any standards.the trouble is they almost always go over the intially quoted budget,take more time to complete than they promised earlier and worst of all selling the cars components as spare parts to others because when someone comes in to their garage asking for some parts the temptation to make a quick buck is too much for them,and they will replace those parts with lesser quality/faulty parts or report them as stolen.there are one maybe two who are the exception.but the price they quote is enough to make you sh*t in your pants.
so most of the owners either restore them at their residence or give it to the professionals of each category such as body work and painting to one company and the mechanical side to another so on and so forth...both of these methods are very time consuming and stressful to the owner.
so coming back to your question,it is a very good business idea to open a professional restoration garage with reasonable prices,the main thing with these guys(owners) is to build their confidence in your garage,because you are asking them to leave their babies in your care.then the news will spread by word of mouth because the classic car community here is a very close knit and an exclusive group.so anyone want to give it a go?