Any question answered...

The wheelset is not flip-flop, you only get one 17t track cog. Yes, the on one cog will be fine. You can get a 20t freewheel cog, they're just not that common as people are massive blouses about spinning. The polo players of the forum will likely be able to suggest a good place to pick one up. You'll regret getting a track bike if you want to run a flip-flop hub. I ride a Langster and it's great, but I ride one of the models that came out before Specialized went full-on tracktrastic, so mine has a flip-flop hub with a freewheel on one side. If I were buying a new bike now I'd probably go for a Fuji Feather or a Charge Plug instead purely for the freewheel option.

Brilliant, thanks a lot!.

So having only one cog, can it still be a freewheel, or does it accept only fixed track ones?It's a shame that that you need to choose only one:(

Unfortunately in evans don't have the Fuji feather on my size, and I'm tied to the ride to work schema with them.
What about the Fuji 2.1 track? it is a bit flashy for my taste, and some reviews out there complain about some details, but I could live with that... Is that one any good? And does it come with a flip flop hub?

Cheers!