Who do you fight against? Does the F-15C have the option of fighting Su-27s, Mig-29s, -35s, etc?
It seems a very limited plane set, if I am reading it right.
Yes, the plane set is pretty limited. (at the moment)
Currently the flyable aircraft are on a flash sale until the 20th.
DCS: F-86F Sabre: $49.99 to $14.99 (70% off)
DCS: Fw 190 D-9 Dora: $49.99 to $14.99 (70% off)
DCS: UH-1H Huey: $49.99 to $14.99 (70% off)
DCS: Mi-8MTV2 Magnificent Eight: 49.99 to $14.99 (70% off)
DCS: Flaming Cliffs 3: $39.99 to $11.99 (70% off) [Comes with A10A, Su-25, F-15C, Mig29A, Mig29S, Su-27, and Su-33. One of the Su's is carrier based and can lift from a carrier in game, although I forget which one.]

DCS: A-10C Warthog: $39.99 to $11.99 (70% off)
DCS: Black Shark 2: $39.99 to $11.99 (70% off)
DCS: P-51D Mustang: $29.99 to $8.99 (70% off)
DCS: Combined Arms: $19.99 to $5.99 (70% off)
F-15C for DCS World: $9.99 to $2.99 (70% off)
A-10A for DCS World: $9.99 to $2.99 (70% off)
Su-25 for DCS World: $9.99 to $2.99 (70% off)
DCS: MiG-21Bis: $49.99 to $44.99 (10% off)
More modules (aircraft) are in development such as the F/A-18 Hornet.