Quick question -- I read somewhere where you can put the sand, coffee grounds, etc., along with paint, when you are mixing your groundwork material (in my case, celluclay). So that when you start laying the groundwork on the base, the texture and basic color is already there, thus negating the need to add them in an additional step. Will that work OK?

Thanks.

For the most part it will be fine.
If you have a thick base to create you'll ultimately waste a lot of the 'mix-in' stuff. So I would build up most of the base, then save for a thin 'mixed' layer.

Lots of advantages to this.
Any gaps are pre-colored, the 'stuff' is already in and mixed, chips and nicks don't have to be touched up.

I'd recommend using acrylic paints.
And yes you can mix your texture materials in with no problems.Just remember the more you add the thicker the paint will become, eventually becoming an acrylic paste like substance.
Cheap craft store paints work well, at less that a $1.00 a bottle, it isn't a major out lay of cash, creativity counts though, and it only takes a few minutes to mix-up a batch. I suggest using cheap brushes, or a pallet knife to apply.
Of course you could wait for the new Tamyia Diorama Texture Paint, but I think you'll end up paying more than a dollar a bottle for something you can make yourself.
Tamyia Diorama Texture Paint

List price is $13.00 a bottle, so Dave's advice is correct...

I've had good results from the cheap 'Poster Paints' which you can find in any toy-store or Arts & crafts store.

Thanks, everyone, for your responses. Very helpful!