First and foremost, find out what size bike fits you. There are a bunch of good Web based fit calculators on the Internet. My favorite is the one in this link:
http://www.competitivecyclist.com/za/CCY?PAGE=FIT_CALCULATOR_INTRO There are many others that can be found with a little searching.
Make sure that you only look at bikes that are within a couple inches of the proper size bike for you. A poorly fitted bike will making riding a painful experience and can hurt you in certain cases. The proper way to measure the bike's primary size is by measuring it from the center point of the pedal axle to the top of the seat tube where it clamps on to the seat post.
I really suggest that you look for a bike from the mid to late 1980's or even the 1990's. The main reason for this is that this was the period of the biggest bicycle boom in history. There are a lot of good bikes still available from this period. Most of them used common components that for the most part are still available. Prior to the mid 1980's, there were a lot of bikes that had proprietary odd sized parts and non-standard threading on their components. It is especially difficult to source replacement parts for older French bikes and older Raleighs because of the non-standard parts that they used. Bikes that were produced after 1985 are fairly standard, and although you might have to go to eBay to find them, there are a lot of parts still available for them.
For a 20 mile commute, you will want a lighter bicycle. I suggest that you look for a bike that is made of chrome molybdenum steel or aluminum. Chrome moly is lighter than carbon steel and is a comfortable ride. Aluminum is lighter but does not flex much which results in a harsher ride. Stay away from high tensile steel which is sometimes called Hi-Ten steel. These are what the less expensive, lower quality bikes are made of.
Moto 700 made some good points. Expect to have to put an extra $200 into the bike once you get it. This will cover the tune up, descent tires, and pretty much any normal replacement parts that you might need like a chain, freewheel or cassette, and possibly chainrings. $200 would be a worst case scenario to get the bike road worthy.
Things that you will probably want to change are the seat and pedals. If the seat is comfortable for you, then by all means use it. But chances are that it will not be the right seat for you. Most bike shops have a program where you can try out saddles until you find one that works for you. Check with your bike shop about their program. Eventually you will probably want to get a pair of clipless pedals and you will need a pair of cycling shoes to go with them. Once again, this is an area where you should consult with your bike shop.
In spite of what Look 566 Rider has said and if you are really a noob, don't expect to jump on your bike and be riding 15 - 20 mph. From my observations, it appears that most new riders start out at around 10 - 12 mph for about the first 6 months to a year, until the muscles that are used for riding have developed and become coordinated working together.After this, speed begins to increase.
