Do you know what external apps I need and which is the most affordable and easy to use. In addition, how do I create CEA 702 captions?
To get a CEA 708 you need Telestream MacCaption and that's not really cheap, or as said PPro but that one is still buggy.
As the others said, ask whether they really need CEA 708 or if CEA 608 will be okay as well.
Additionally ask whether they will accept Apple ProRes - most stations will do.
If they insist on MXF ask whether they accept a sidecar file with the captions in 608.
If they don't, use a service to create an embedded file. Probably it's easier and cheaper rather than buy/rent the above mentioned apps.
Inside FCP you can convert SRT or ITT to SCC (CEA 608) in case you have no .scc file. FCP does a pretty good job showing caption errors and making suggestions how to fix.
While the CEA 608 specs say 32 characters per line this is only true if there are no colors, style or special characters. So some stations say they accept only 27 characters per line. Ask for this as well.
If positioning is important for you check the FCP manual about this.
Some time ago I made a generator for this. It's a visual helper for this: