The developer recently commented on another request that was similar, basically about adding additional controls to a field type http://support.advancedcustomfields.com/forums/topic/get_field-the_field-return-escaped-value/#post-32413
For something like this I would create several fields, not just a single field, that’s if I wanted to give the editor the choice, and to be honest, I’m not sure I’d give the editor these kinds of choices, I might just confuse them.
1) Link Type: Radio internal or external
2) Page Link field conditional on 1
3) Url field conditional on 1
4) Check box field to set if it should be opened in a new window
5) A text field to add the link text
6) A text field to add an alternate title attribute for the link
