I don't kinda agree. You don't have to be an expert in cars to
be a boss or investor of a company. I rather you be alittle
knowledge in modding cars and be expertise in running your
business. Why? COS its more important in managing your business
well and smooth rather than concentrating on fixing cars when your
workers are suppose to do those. Whats the difference if you are
doing your worker's job and working as a mechanic in a garage? And
then, who is going to manage you business?
Defintely, a little knowledge do helps but you can learn while
running your business. It's more important to hire the right kind
of workers that can be trusted. It's hard but you have to lose some
to gain some. That's part of a risk you have to take.
Research reseach and more research should help alot. Concentrate
more on your plans rather than learning from basic. It's a waste of
time. Learn while you work. Seek advice from professionals. Get
accurate details of where to get the best for your company.
If you need some help, i suggest you getting inspiring books on
starting a business, i recommend "Clueless in starting a business"
by May Lwin, Adam Koo. Its written and based local. lots of
inspiring storys and advices for those who wants to make big in
starting their business.
I'm actually planning to startup a small business and a garage have
actually been in my list. Unfortuantely, a garage is way too costly
for me to handle.
Lastly, have the passion. Go for what you want and stop wasting
time. Go do your own research and stuffs.
That's my piece of advice. Listen if you want to. 
