So far, there's a number of different paths the different Cougar owners have taken. Common mods so far are suspension and rims. Anything else and you start having to deal with some sort of customisation because of the sort of car we're dealing with. Audio would probably be next, a few of us have kits, with pieces being sourced from the US.
Not many with engine or exhaust mods, these either have to be shipped as kits from the US or custom made. Rims are fussy with their stud pattern. Not sure how viable shoe-horning a Mustang V8 would be, there's not a lot of room in the engine bay... and FWD V8? hahaha!
For some of what I've done with my Cougar...
http://www.cougardb.com/car.php?id=Abraxas
Dan