How much power do you want? You choose a turbo based on a target power figure you think you want/will get. For up to 270bhp use a 2wd cossie turbo, for up to 350bhp use a T34, and so on..

If you really want to join the internet craze and get a GT.. then a GT25 for say 225bhp.. GT28RS for 300bhp.. and so on..