DCS World is free, includes the Su-25T for free. And you can download anytime to try it out at http://www.digitalcombatsimulator.com/en/downloads/world/dcs_world/
Once you've gotten used to it and checked it out, if you want to access other airplanes this is what you'll need.
A-10C, BlackShark 2, Huey, Mi-8, P-51 are added modules, that require NO pre-installed software. These are also very well simulated and very detailed.
Flaming Cliffs 3 (FC3) has : F-15, A-10A (not the C, but A model), Su-25A, Su27. Su33, and three variations of MiG-29. This does require owning Lock On to be installed like cichlidfan said above. These are partially simulated...like the "easy mode" type of aircraft for casual simmers.
The Su-25A and A-10A from Flaming Cliffs 3 (FC3) are sold separately. But I think it is more practical to just buy FC3.
Hope this cleared things up in detail.