Save the money until you can afford 50' coil. keep an eye on ebay- i saved about 15$ even after shipping by ebaying the copper. make sure you look for cleaned and capped copper tubing.
the 20' will work, yes, but a 50' will work MUCH better and will allow you room to grow. technically, you can extend your 20' section through the use of compression fittings, but there will be SIGNIFICANT hassle in doing that. You'll be much happier if you wait until you can afford the 50' coil and don't have to swap out later.
you'll get a colder chill now as well and as a result will precipitate out more break material.