Ciaran
The classes for the Impreza's are pretty much determined by what you do to the car. if the car remains road legal and is not stripped out etc it would be entered in Class A3 but if you stripped it out and extensively modified it then it would probably run in the modified B3 class. That is a bit simplistic but where you place your car really depends on what you want to do to it. There are a lot of rule changes happening this year which will see some cars become inelligible for for A1, A2 and A3 due to the modifications carried out.
The club you join is not necessarily determined by where you live but by what you want from your club. There are a number of excellent clubs in the central belt like East ayrshire car club, Monklands car club and Lothian car club. Further north you have ADMC, GAC and Caithness car club.
As anyone who has competed in the championships will tell you this is not a cheap hobby. With travel costs, entry fees and accomodation costs never mind the cost of building and maintaining the car, helmet, race suit etc etc it is something that you need to properly research before diving in and pulling your car to bits.
If you do decide to do it there are plenty of people around to help and advise