There are plenty of good backgrounds, cultures, destinies, and heritages. What works best depends on what sort of build you are aiming at. I don't think there's any in LUA5E that are substantially better than all the others. The Tyrannized culture with resist fire could well be appropriate for a desert dweller that grew up in harsh circumstances.
It sounds like the main thing that Metalith is looking for now is a background. For a Cleric, it depends whether you're going for a build with medium armor, so no point having Dexterity above 14, or no armor nor shield with Armor of Conviction for good AC. What background choices to choose from depends on what ability bonuses you need:
- With a medium armor build, I'd want to start with Wisdom 16, Constitution 16, Dexterity 14.
- So you want a background that can give +1 Constitution and +1 Wisdom.
- With an unarmored build, I'd want to start with Wisdom 16, Constitution 14, Dexterity 16.
- So you want a background that can give +1 Dexterity and +1 Wisdom.
Acolyte, Exile, and Hermit backgrounds work for both of these, as they give +1 Wisdom and +1 to another ability. Folk Hero, Guildmember, and Outlander could work for the medium armor build, as they give +1 Constitution and +1 to another ability. Maurauder could work for the unarmored build, as it gives +1 Dexterity and +1 to another ability. I've skipped LUA5E backgrounds that seem particularly inappropriate for the character as described.
If you weren't coming from the desert, I'd suggest going with the Warpriest archetype and heavy armor. For that, I'd want to start with something like Wisdom 16, Constitution 15, and Strength 15, aiming at 4th level to get the Tenacious feat for +1 Constitution and proficiency in Constitution saves to help your concentration. You need Strength 15 to wear the heavier armors.
For an armored build, put your holy symbol on your shield, and you can cast spells with V,M or V,S,M spell components without a free hand; but not V,S - as that does require a free hand unless you have the Battle Caster feat.