CAPI makes a really nice clone of the 550A, the BT50. Sounds closer to originals than the current API offering, there’s was a shootout on realgearonline.
I also like the Electrodyne preamps. Very nice transformer-coupled DI as well.
If I were getting a first bus compressor, I would prob not do 500 series.
I would either have a tech build and calibrate a gssl with sidechain options, or get the Overstayer VCA, or a class A 33609/diode bridge style compressor like the Heritage Successor (because seems like good value, haven’t heard) or BAE 10DCF pair (a lot more money but sounds good).
The RND 535 is very nice, though, sorta halfway in flavor.