The separate transponder way would not be accepted in other countries as its not 260B
Only 260A.
I have asked them to quote the cheapest install even if it means ripping it out before I sell overseas .
If you're prepared to rip it out if the citation sells overseas, then what's the problem of having a 260B transponder? If you keep both a mode C and 260B transponder in the aircraft you wouldn't even have to worry about flying it over in the states for the next 5 years, so I fail to see the issue?