Money is talking on brake parts. The dealer have interest to replace rotor quite often. In practice, too thin rotor can be hazardous if the caliper piston can move out and leak with wear out pads. In this case, if the spec call for 24 mm minimum, this is probably the safe minimum to avoid this trouble.
Also, thickness of the disc is important for the rotor mass itself. The mass is important to accumulate heat during braking. More the mass is, and less the rotor temperature will rise. Now, the Lotus have quite big disc for the car weight, and in street, good luck to create overheat of the pads. The car is able to track with standard components. If you don't experiment brake pads fading during your usage, this mean that the discs are heavy enought for your application.
Brake disc price difference? Not an easy one to answer. Typically, more expensive disc are welcome to be more stable and stay straight at high temperature. Also, depending of the brake pads you are using, the disc must not wear out too quickly. And also more expensive disc may have better air vans to extract heat after braking to cool down quickly. If you use very agressive brake pads, harder disc are welcome. If you use standard pads on street and run smouth and not create high temperature, you can use cheaper disc. If you use agressive compound brake pads like Raybestos ST-43, hard disc are more than welcome (I am using Alcon disc $500 each for 3 years now street and track with the ST-43 and more than 300 hp). For the sector disc, you can have 2 pieces discs, it save weight and since the hub is an aluminum and not part of the disc itself, the disc have less tendency to curve at high temperature. One piece rotor is good for low temperature, and 2 pieces are welcome for high temperature, and save few pounds of not usable mass.
The car have typically too much braking bias on the front, big brake in front just increase this already wrong brake bias.
A lot of options down here, and all make sense for different application.
Elise 2006, white, K20 turbo GT3076R, 395 whp
List of mod: I am just stupid putting that much money on that car!