My specs would be like this:
Red Bull RB9 chassis
Renault RS25 engine (1100hp)
Mercedes PU106A Hybrid Systems (200hp)
Bridgestone 2010 slick tyres
Exhaust Blown Diffuser
Double Deck Diffuser
Active Suspension
Various other aerodynamic ducts and deformations
Active two way electronics so that engineers can change maps for the driver.
Weight at 705Kg
Target Lap for Monza: 1:10.000
Target Lap for Spa: 1:32.500
Target Lap for Monaco: 1:08.700
Target Lap for Red Bull Ring: 54:350
Anything that develops about 7.5 to 8 tons of downforce and not the present leading at 3.5 to 3.7 tons of downforce.
However, the car would either have to be driverless or have the driver in a G Suit as Au Rouge would be taken at 340kmh flat, and at the speed trap at Monza you would be pushing 450kmh with all that horsepower and drag being dumped from aero devices. Breaking for Paribolica would only be about 35m in total as Aero would be powerful enough to almost flat line that corner.
The budget for one car would be at about €400m. The present budget for Ferrari for a single years racing in F1.