That's easy.

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.