You need (cunningly) a Self Drive Hire Insurance policy, and the guru is your man for this.
It covers two things:
1. the vehicles while at your premises and when your own drivers are using it.
2. the vehicles when they are on hire to your customers.
To be honest if you yourself are under 25 I think you are going to struggle to get the insurance, let alone a low quote.
What you need to do, via an insurance broker, is to convince the insurance company of the following:
1. you have are actively controlling the risks associated wit the business. i.e security of the vehicles, vehicle tracking.
2. you are controlling the risks posed by your clients. i.e credit, identity and licence checks.
3. you understand and have experience in the self drive hire business.
If you cant do the above 3 things then you will not get cover for the business
Another couple of points you may not have considered are:
1. newer cars cost less to maintain, are more secure, and will have a manufacturers warranty.
2. when a rental company buys NEW, or Vat Qualifying vehicles, they can claim the full VAT back on the vehicle.