Welcome to the hobby! Sounds like you're on your way to to nice clownfish tank. Aquascape looks good (though the picture is a little dark... hard to tell exactly). But you can always rearrange it later if you don't like it.
A 20 lb bag of sand should be deep enough in a 20g to produce a deep sand bed. I've only got an 1-2 inchches and I can see bubbles of nitrogen coming up through it. I should add more sand though. 2-4 inches is a good depth in my opinion. try and add a couple of sand sifting snails. They will help keep the sand bed nice and healthy.
Jager heaters are good. I've had one for 10 years and it still works fine. A 150watt might be too much for you though... that's what I have in my 60 gallon! Maybe try 75 or a 100, then if it does get stuck on, it won't cook your tank quite so fast.