It's a pretty saturated market already. I know of at least 4 in the "greater Brisbane" area, at least a half dozen more around Oz in all the major cities.
Biggest problem I see with most is why I generally buy from NPD in Florida. If more than a one-off, its generally cheaper to buy in the US and air freight. Even for things like exhaust systems and other large items...
Look at the current market, look into your sourcing/storage/handling costs and do a realistic assessment to see if you can compete against the US/air freight and the multiple local vendors who have already been doing it for years and have street cred with the hobby.
You are correct, there is competition. However we are talking Classic cars, like everything else it will be a case of suck it and see. I am still looking at the licensing side of things etc. I believe a good car will sell itself, If I have access to that, I will have something competition cannot provide creating the 4 P's, Product, Price, Promotion and Place, remembering the Marketing concept is that we should not try to help ourselves unless we are trying to help someone else.
If I have what a person wants they will pay a fair price to obtain it, else they will pay nothing.
Perhaps I will find a way of selling something i do not own, on consignment, my investment will consist of meeting supply with demand and in the event that has not happened I will have lost nothing.
My thoughts come from study of marketing in my Bachelor degree and selling in my experience as a salesman, in the past. I believe I will benefit from my time spent working with my brothers in my youth, in their panel beating and spray painter trades, with good administration skills gained when I became an accountant.