$ + how much you value your time. Business such as dealerships, peformance shops etc. are not started over night.
Figure your overhead might be in the 20k-40k range (taxes, morgage/rent, utilites etc.)
Monthly costs flucutate. Some months may be better than others, but the bills always come. Also may want to consider how large of a staff your going to have. Manager, techs, secretary. This is just a few things to consider to "get started" must talk to vendors for supplies because it is hard to sell nothing. Unless you plan on selling your good looks
Now, I am not trying to put any ideas down. Just trying to say there is no exact number, as there are many factors to running each business and got to remember the unexpected curve balls that get thrown in the mix. If you want both an exotic dealership and this, I say have 1-3 million saved and ready to lose.