I would not go to 600. I live an hour from Jeff Jordan and RPM. When I bought the car it already his clips and some other stuff. It was natural to go to him.
His cars win races every weekend, but so do a lot of other guys.
These cars are not very comfortable to drive, they take some getting used to. A lot of the top teams rent cars. I think about $500 a race (Less than you pay for tires on your LM). You will get a top car with a solid setup, show up with helmet and drive. Give you an opportunity to see what you think.
Where are you located? If your near me I could point you in the right direction.