Don't forget to use something to QT in. A cheap 10 gallon with a really cheap filter would work. One of the most discouraging things for people getting into the hobby is to finally get their tank cycled and buy fish then end up getting ich or some other parasite in their main tank just to have it set for a month until the parasite runs its life cycle and dies. You will have to test your water for ammonia, nitrites and nitrates so get a testing kit. If you want to buy your clowns a week or two after you start to cycle the tank they would be ready to go in once its done. Yes, you will be doing frequent water changes however you will know that they are parasite free.
On the equipment, with a 20 gallon you can't go to crazy but I would buy the best you can afford because once you figure out that you are addicted, you will want a bigger tank!! LOL! I think most of us have done this. As far as lighting goes, IMO
, I would keep it cheap until you can get some really awesome lights for corals...at some point, you will want them...unless you are planning on having an anemone but quite honestly you need to wait for your tank to be more established and gain some knowledge on water chemistry etc. Maybe some would disagree. The biggest issue with them is tank stability, water quality, and lighting. You want as little fluctuation as possible.