It seems to me that the color scheme is the most important mechanical factor of modern SS/FG bikes.
Flip flop hub is big bonus if you are just experimenting with fixed gear for the first time.
Then the gear ratio should be considered, but you can always change the cog, it's not expensive to change.
If you buy a bike that is designed for use as a SS/FG you don't have to worry about it having the wrong type of dropouts.
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My advice is free of charge and of respective quality.
1982 Miyata 912
1998 Wheeler 5900 with front and rear air cushion suspension
2015 Canyon Spectral 7.0 EX