IS-BAO is certainly not a requirement but may help you become knowledgable with compliance requirements.
As Chriskcmo said, you need the permanent reg, not a pink. You can have them hurry it up(supposedly) if you issue a form stating intentions of int'l flight.
LOA's: MNPS, RVSM, P-RNAV/B-RNAV, RNP, ADS-B Out(if equipped), MEL
Other: SMS, Link 2000+, TCAS with 7.1, 8.33Mhz radios, both crew rated in the aircraft, Insurance that shows appropriate coverage(many EU countries have specific minimum liability limits(a CL30 is about $150M but that number changes with exchange rate so go with $200M)), Save yourself hassle and get a Class I medical(Class II meets ICAO but they get confused)....all I can think of there.
Make sure your company shows up as operator under the AGHME results(required every 2 years or 1000 hours, whichever longer)
ETS - you do that ex post facto. Have fun with that.
Make your FSDO aware ASAP if you want the appropriate LOA's to be completed in a timely manner. They likely won't do anything until they see the C of A and see the registration show up in the OKC database with your info.