So far, it looks like the traditional Highland rain outfit is:

A Barbour wax jacket, suggested by Cajunscot and seconded by JerseyLawyer and possibly Figheadair or Canuck of NI,

Alternatively, a tweed or wool inverness cape, suggested by JerseyLawyer and seconded by Riverkilt and Starbkjrus (Mr. Antony Inverness Cape ),

A fore 'n aft or deer stalker, suggested by Jock Scot and seconded by Starbkjrus (seeking a deer stalker source),

A TOS, suggested by Figheadair,

Wellies, suggested by JerseyLawyer and Figheadair,

A heavy duty umbrella from Swaine Adeney & Brigg, suggested by Jock Scot, umbrellas seconded by JerseyLawyer.

One item that seems to be absent in the discussion is gloves.

There are other suggestions, however, it is unclear that they are traditional Highland attire.