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here is something that will shock you, initially when I brought the NBD, it came with a surely free hub, I got the wheels rebuilt and converted to fixed, but after nearly killing myself riding fixed around the country lanes I decided to go back to SS again, but the surely fixed hub is still on but with a SS freewheel instead of a fixed wheel cog.
So long story short, get a surely fixed disc hub and you can run both single and fixed gear.
Here is my commuter which I think is pretty similar to what you're building, personally for me I would use SS over FG, but then again I'm a old fart who started to appreciate coasting: