Super idea! I've had a Weber Smokey Joe Kettle grill for close to 30 years! It's the one I use when I want to use charcoal briquettes. I have used the lid of an old propane grill as a planter in the garden as well as using old boots I wore out!

My main suggestion is to go to your local Lowe's (my favorite place!) and ask for the garden specialist and ask the same questions so you can get plants specific for your area. I'd definitely use the "moisture retentive" potting soil as keeping a container moist is a concern as they will dry out faster than the ground. Know where it's location in the yard is going to be in regard to light exposure, full sun or shade.

When I do planters I like a little height in the middle or back side down to trailing on the sides or in the front. Good luck with it and please post pictures!