Adding a dissenting voice here. You do not "need" a mage / cleric / any one role in a game based on the 5e system. The difference between a party of 4 and party of 5 is massive and will trivialize most encounters in a way that cannot be fixed by simply adding more HP. A party of 6 in this game would be preposterous.

You can heal by eating food (let alone by resting essentially at any time for free), and the game is much more combat focused than a traditional session of D&D where skills and utility spells are not nearly as significant. There is much less need here for a balanced party that covers all possible bases than there is in D&D 5e, which is already very forgiving in terms of party composition needs.