A friend might get a Brompton, and would then want to install a double chainring.

Based on the above, I will recommend getting the LitePro B-Series Braze-On Adapter For Birdy instead of the K-Series I've been happily using for a few years now.

As for the front derailleur, and an alternative to the Shimano Tiagra (FD-4600?) I used… are there yet cheaper options I could use? I'm just curious.