@Bizjetlover...
"6 million"
"Total costs on a Buenos Aires miami Buenos Aires no more than 50k"
With those numbers don't even think Global Express or G5/550 way too expensive.
A return flight like you mention above on the big planes would be double your budget.
I don't think you could get a 605 with a 6 million dollar budget but I could be wrong.
I remember that when we operated a GIV SP it would burn at least 25% more gas than the 604 and its operating cost were overall higher but I don't have the details to this.
Also remember that both the 604 and GIV series aircraft are getting old and may need expensive maintenance and that their avionics will need to be extensively upgraded in the future to meet the new norms (CPDLC, ADSB and so on). This would also apply to Falcon 2000s.
If you could find a 605 with the latest avionics, perhaps that could be a good aircraft for you.