It sounds like you've got everything in very good order, good job on all of that and consider me impressed.

As far as the things you still need, most of what you can find at the nearest petstore. If by background you mean the kind that goes on the outside this can be as simple as some colored poster paper or a fancy aquarium/terrarium kind they'll cut to size at most big chain pet stores. For a hood, my husband used to use a zilla desert strip fixture for our bearded dragon and that worked pretty well (we've moved on to a custom terrarium build with a strip fixture we bought from lowes). The day/night timer we use is the Zilla power center, also sold at the pet stores. Just make sure you actually get the programmable one with day/night sockets, as they also sell a regular power strip without a day/night cycle. For the hygrometer, I would go with the combo thermometer/hygrometer with the probe, but I'm pretty sure anything accurite has to offer will work pretty well.

The stand I have my 20 gallon tank on currently is just a regular entertainment stand from Walmart that I bought for a different purpose a while back for like 30 dollars. It stands about 22 inches tall, and since I'm only 5'1" it's a perfect height for me since I don't have to bend at all. At this point, whatever you like is going to be totally fine since you seem to have covered all the bases pretty well.