A nice mixbuss chain would be my priority - the api is a good choose that would also be good for tracking, but any stereo compressor that you like also fits the bill - manley, ssl-style, and so on. Next priority would be a mixbuss eq - I own and like the hammer, but if you have the cash a massive passive would be a more versatile option and also good for tracking. Gml if you like super clean. Api less flexible but a good option.
On top of that, I'd look at some good mono compressors for choice sources - 1176, distressor, or similar.
Converters - all the above will make more of a difference. Once you have that in place, converter upgrades might be appropriate.