I wouldn't worry much about the clean out. I assume some lava rocks in the bottom I would dump out in a pile & set aside with any ash. Clean it out best as you can & I would spray paint inside with a rust resistant sealer. You might also want to drill several drainage holes in the bottom.

Then I would put the rock back in the bottom, get planter mix with fertilizer initially in it & I would mix any ashes into that. Put it on top of the rocks & fill to about 2-3 inches below the top of the BBQ & ready to plant. The holes in the bottom of the BBQ & the rock create drainage for the plants....critical for healthy plants.

Succulents work good but use sandier potting mix for them. Usually potting mixes in the nursery specify what plants they are for. I would still mix in any ash....just adds more nutrients & no reason to waste.