I would say you need either a TR4,5 or 6
Porsche 911 pre 89 (targas are cheaper)
E type
Morris 1000 soft top ( not a sports car, but it would be a popular car to rent out) or orginal Beetle Soft top
Maybe a classic camper (there is a company renting out classic VW vans down devon way)
Rolls or bentley
I would not limit yourself to Sports cars, its a business, not everyone is as excited about them as us!
The auctions are going to be softer on price than trade or private sales, but you need to know the likely problems are going to be.
Good luck.