So what rank are characters allowed to progress from basic to intermediate/advanced careers?

For example I have a rank 1 High Elf Envoy in my party who is wanting to take Ambassador when she gets her first career transition. The only real difference I can see between the Envoy and Ambassador careers is that the Ambassador has the Intermediate Trait and gets to be Noble, which seems likes its a natural extension of the Envoy Career. However unlike other advances it doesn't require you to actually complete the Envoy career first. So does this mean she can move to Ambassador at rank 2 once she completes the Envoy Career? Would she have been able to just take Ambassador at character creation and just skip Envoy altogether? Also does the change from Basic to Intermediate require an extra advance as its not a shared trait?

Finally whats the rank needed to go from intermediate to advanced careers? Rank 3?

Normally you complete a career by spending the 10 advances on the general career advances (plus any spends on open career advances). After that you spend one XP on dedication bonus, and then you start spending XP for career transition. So often you change careers after spending 12-15 XP or so in your current career.

Not completing (and dedicating) the Envoy career before moving into Ambassador means you loose the Envoy career card, you also don't get free specializations on skills trained during the Envoy career. Plus dedication lowers the XP cost of career transition by 1. So in almost all cases it is much better to complete the career before moving into a new one.

WFRP has no fixed career progressions, you could go from basic to another basic career, or from basic to intermediate and then back to a basic career.
Per RAW intermediate, advanced and basic are just traits, which doesn't have a rank requirement. The traits determine how much XP career transition cost, so moving from basic to basic could cost less XP than moving from basic to intermediate for example

But you could also rule that the requirement should be:
Rank 1 - Basic
Rank 2 - Intermediate
Rank 3 - Advanced
(Rank 4 - Epic)
(Rank 5 - Heroic)

The last two traits are introduced in Hero's Call I think.

Ok, think I've got it, thanks!

Hero's Call book addresses this question as an optional rule. Check there. I don't have the doc currently with me and cannot provide oage number.
But I remember that the suggestion there look very much like k7e9 wrote.

Yes, that's probably where I read that optional rule now that you mention it.