British thermal unit, an British standard unit of energy. One Btu is equal to the amount of heat required to raise the temperature of one pound of liquid water by 1 degree Fahrenheit at its maximum density, which occurs at a temperature of 39.1 degrees Fahrenheit.

If it's just one room you're looking to cool, anything over 7000 BTU will do the trick.
You'll kick down about $200 to $300 depending on the model and the features. Some come with remote controls which come in handy should there be a piece of furniture between the AC and you.

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unit for measuring heat quantity in the customary system of English units of measurement, equal to the amount of heat required to raise the temperature of one pound of water at its maximum density [which occurs at a temperature of 39.1 degrees Fahrenheit (F) ] by 1F. The Btu may also be defined for the temperature difference between 59F and 60F. One Btu is approximately equivalent to the following: 251.9 calories; 778.26 foot-pounds; 1055 jou ...more