Building a city/trail bike with a CX frame, what should I consider?
FWIW, Fixed gear off roading is possible if more difficult than needed. This site has lots of useful info. 

http://63xc.com/

I ran my Surly Cross Check as a fixed gear scorcher for season or two and it's way to make easy trails hard. It's definitely not for everybody and the downhills can be brutal. 

I'd encourage you to go for disk brakes on the front if at all possible and you definitely want a front brake. I also found it very useful to have a back brake as well. Getting any kind of "modulation" in braking with a fixed gear was beyond my skill set, skidding on dirt is a really great way to crash.  

Also, an eccentric rear hub will allow you to use a fixed gear wheel with just about any dropouts. White Industries ENO hub has worked well for me.