Undertaker looks grand. I'm pretty sure the soiled dove is not from minifigs. On the terrain front, since it looks like you are going for the desert southwest, check out both Peter Pig (great range of old west 15s) and Stone Mountain Miniatures here in the states, both have packs of cacti plus you can get things like horse troughs and tie rails and SM has some suitable southwest structures. At Peter Pig, check out there scenics range for a variety of items, including dead cows of all things!
As for terrain generally, pretty easy to make. And if you are going for Arizona you don't need to stay with dry browns and tans, lots of green in that state's portion of the Sonoran Desert. (I think it's part of that anyway). Also, check out gift shops for small pieces of use; I found some really nice cacti I'll be using - though admittedly that was on a trip to Arizona!