An Indy car is a based on a tub with specs provided by the IRL. I think it's the same platform that they have been using for years. Up until recently, the chassis was purchased from Tony George and he held the contract with Dallara (or whoever he approved). The front and rear aero parts are made by others however they have to fit the envelope rules that were basically the maximum dimensions and position of the wings. The only innovation there is the giant sideplates that are on the rear wings for advertising LOL.
FI cars, on the other hand are a complete "clean sheet" of paper design with the front and rear aero package completely integrated. The design of the sidepods on an F1 car probably have more time and effort that the starting grid for the Indy 500.
Indy car is bigger, heavier and slower than an F1 car. FYI: CART is history. All the teams supposedly merged with the IRL. However, since you mentioned defunct racing organizations, a 90's Indy Lights car would probably give a current IRL car a run for the money. I talked to a sprint car builder in Indianapolis and said an old Indy Lights car can be had for under $20K.
