[VF711Merlin]

You will need to install " Unified Install 1.1 to get newer mustangs", not sure if you need to do a clean install of the game itself and patch it with 4.08, 4.09, then 1.1 unified or HSFX if you wish.

I use HSFX install myself, but it your choice.
Anyway I strongly recommend a fresh intall of the game before you do anything.