20% depreciation? Will anyone want to rent a 5 year old? You target audience will want new, fast cars.
£1500 maintenance across three performance cars? Dream on. The target audience that you mention will all drive these cars like utter bell ends. £1500 won’t even cover the servicing costs. Start looking at break pads, alloy refurbs and then onto clutches and gearboxes and the costs soon start racking up.
Also, your costing is massively flawed by working out the cost over 365 days to arrive at your hire price. The cars will not be rented for 365 days per year. Start with assuming that your cars will be rented out all weekend, every weekend if you are lucky.
I guess what I am trying to say is that there is no money in this. The only reason there are so many performance car hire businesses around is so that dealers can launder their money.