It is best not to get an anemone until your tank has been stable for a few months. Clowns don't need an anemone. Anemones even when they are happy can decide to tour the tank and end up messing up the whole thing by getting chopped up in a power head. This is really a good read Reefs.org: Where Reefkeeping Begins on the Internet - Keeping Anemones
For 2 clowns I would get a 20 or 30 gallon tank because when they start figuring out who is dominate it may take awhile and they might need their own space. I don't care for bare bottom tanks so sand and live rock it can be base rock and will actually turn to live rock. You will probably like to have a light. A heater is a must unless you keep your house about the same temperature day and night. You will need a refractometer it is more accurate than a hydrometer. For food I use formula 2 and rods food so my fish get a balanced diet.
I forgot you will need a good liquid test kit like API.
About the life span of a clown my lfs
had one for over 25 years that died recently and they have one that is about the same age that is still alive. Anemones usually don't live a year in a tank unless it is one that is one that has split off another one then it is possible they live for 5 years or more. I say 5 years because I know someone that has a rose tip that is that old who wanted to sell it.
Now after all that information you will need to read up and understand about cycling a tank without fish.