Since I'm somewhat tired I didn't really look at your post very good so if I missed or misunderstood something, Let me know. I would explain more but I am really sleepy, its been a long hard day for me
Anyways
Well first you will need Version one of San Andreas. If the disk says Second Edition, you'll need to look here
Downgrade
To add mods get the IMG Tool. Here is one Click Here and follow the instructions in the readme.
To add save files. Just put them in your GTA San Andreas User Files folder which should be located in "My Documents" on your computer.
To type in cheats, Just type them on during gameplay.