I would say be open-minded about which one to go for. Although there are slight visual differences between each, theyre mechanically the same car. I recently bought mine and was really after a Williams 2 as they look more modern than the original Williams (which has Phase1 trim), yet dont have the sunroof or ABS of the Williams 3.
But, the Willy is a very rare car! When it came down to it, I was buying on mileage and condition. Believe me that you dont have much choice of mint cars, let alone of a particular Willy, 2 or 3! I settled for an original Williams (no. 123) as it was a genuine low mileage car in great condition. Its a bit less modern-looking and noisier than a Willy 2 or 3, but it is one of the originals and its still a beautiful car. The 449 Sports Blue paint is still stunning after all these years and catches the sun very nicely (goes purpley).
Youll be getting a classic whichever Willy you go for!
Briefly:
Williams The original homologation special. 400 numbered RHD cars. Phase1 trim with no sunroof or ABS (because rally cars dont have them), manual mirrors and less soundproofing. Colour: 449 Sports Blue.
Williams 2 Phase2 with more modern trim, soundproofing and electric mirrors. Otherwise the same as Williams and no sunroof/ABS. About 400 RHD examples.
Williams 3 Another Phase2. As Willy 2, but with sunroof, ABS and brighter Monaco Blue paintwork. About 400 examples.