I found a nice index of careers here: http://www.apathyblogs.com/warhammer/index.cgi
In the ‘Type’ column are two values – basic and advanced. Basic means you can start on that career, and ‘Advanced’ means you will need to go through one or more careers to get to it. For example Master Wizard requires that you have been a Journeyman Wizard, and before that an Apprentice (which is a Basic career). Some career paths such as wizard, priest and chaos warrior have a clear progression through the classes, others are less restricted.
If you have a career strongly in mind that isn’t covered by the books it is feasible that we create one.
Limitations on gender, species and nationality can be navigated through roleplay. For example if a player has their heart set on their female character becoming a Grail Knight then through roleplaying (and a minor social movement within Bretonnia) this path could become open to them. These limitations will usually be in place for the first career however (except in cases of adoption or childhood disguise).