Try San Fernando (SADF) in the northern part of BUE, now they have just (finally) reopened as intl GA airport, though intl flights are subject to a 10 min flow control, PSA, Aduanas, Sanidad de Fronteras, AA2000 are together and close by, you can solve all your stuff in around 45 mins or less.
Don't even think about Aeroparque (SABE), they might charge you up to U$500 only to make the airport intl since is domestic only, and you mus do it trough the FBO's based there in case you want. Last time i went in there was for the Eliminatorias soccer game, and we ended up paying around u$3000 for handling and stuff.
Ezeiza (SAEZ) might be a good alternate but customs processing might be quite lenghty since everything is too far away.
( may be you want to check if Rosario (SAAR) fits your trip too, easy one stop for paper handling, Avgas and Jet-A, and no Intercargo
)
Good luck
