Is it possible to have an app meant both for internal employees and external public in the App Store?
You could reply to Apple stating that the app actually is not used exclusively by internal employees - but rather is made available publicly for marketing/recruiting purposes. I have often seen that Apple's initial request for further information and clarification results in them approving the app update afterall.
However, if you are forced to switch to the enterprise model - don't worry at all. You can still offer access to potential employees. You would then just host the app on your own web site (or any web site you have lying around really) - and just send the potential employee a link to install the app.