Install the game inside STEAM (Add game) - no DVD need - just the license key.
Then copy the folder "CLiffs of Dover" from STEAM/SteamApps/Common/ folder, past in (example)
C:\Cliffs of Dover (o D:\ if available - better because "format C:" don't affect the game).
Download the TF MODs and install over this copy, one be one.
You end with "vanila" game untouched in STEAM folder (consider a "backup").
And TF MOD version outside STEAM folder.