You will need two rear wheels because road gearing will not suit CX. For road I ride 42×15 which is geared for 19.6 (275 revs per mile – I count them) mph at 90 rpm. I led a club ride on this gearing last week with an average speed of 20 mph. You spin like me, and this is a fine gear for 21-22 mph. It is a drag into a headwind though.
For CX you’ll need lower, i’d guess 42×17 or even 42×18. You can scale speed directly for gearing, but it looks like about 16 ish mph. On my SS MTB running 32×17 I spin out at about 16 mph (26″ with 2.2 tyres).
The benefit of a second wheelset is you won’t need to swap tyres or sprocket. Of course chain length might be hard to accommodate.
One last point, riding SS CX, you’d be best with adjustable sliding dropouts, because swapping out a back wheel quickly is harder with horizontal dropouts – vertical will retain correct chain tension.
EDIT; as for chains, I ride with a cheap SRAM. No shifting, so no point with anything fancy. I don’t need the strength of 1/8″ or half link. I did buy one, buy oh it was so heavy!