The Wordpress functionality really works quite well in this regard. You don’t have to choose between internal and external, nor does it assume one or the other.
If you’re inserting an external link, you’re generally not going to be typing it, but pasting it. If a link gets pasted in, don’t bother to search. If words start getting typed, run a live search for suggestions.
Not sure it would need to prevent one-boxing either. Presumably it already puts the whole link in if they don’t add the optional title. Isn’t that how a one-box is created anyway?