I too take concept of Mahdi with a pinch of salt. Its true that Ibn Khaldoun was not a traditional scholar but he is a well respected historian and his views on history has gone long way in shaping sunni beleifs. Even the scholars who beleive that traditions about appearence of Mahdi are true like Hanafis, also state that beleif in him being Mahdi is not required for Eman.
that doesnt make any sense. Let me put in a very simply way.
Hazrat Muhammad PBUH gave us the good news about Imam Mahdi AS's appearance.
In Quran it is said to obey Allah and His Messenger, whoever disobeys His messenger, is disobeying Allah.
You not accepting Imam Mahdi's appearance is disobeying Prophet Muhammad PBUH, which is disobeying Allah.