Fixed gear bikes can have it both ways with a reversible wheel that has a fixed cog and a freewheel on the other side. Here is an example of that: http://www.performancebike.com/bikes/Product_10052_10551_1072944_-1_400306_400001_400317 take note of the specs/ wheelset.
Only 700c wheels are going to work on most of these unless it the bike you get takes a 650 and the two are not interchangeable! You could very likely just order the rear wheel on this bike...then you dont have to worry about it.
