You've got a few pounds on me, but not much. I'd probably go 24f/28r as a nice race-able build. 32 would be even more robust. It really depends on how hard you are on equipment and what your local courses are like. Are they mostly smooth? Lots of dips and ditches?
I guess I should have mentioned, despite being lighter than the OP, I am running 32 3x on pretty much every CX wheelset in the quiver. So to Justin's point, going higher spoke count has very few drawbacks, especially if you prescribe to the school of thought that rotating mass is the biggest "feel" factor in a wheel. Same rim and tires, different spoke count isn't going to significantly impact your rotating mass numbers.
I am expecting a set of Sapim Race spokes to arrive tomorrow. They will be laced 32 3x to a set of Ultegra hubs and Velocity Major Tom rims. My other current wheelsets are Velocity A23 clinchers. One set 24/28, the other 32/32. All will be wearing CX tires this season.
Hardly any downside to erring on the side of higher spoke count in CX. Weight penalty is marginal, as is aerodynamics. I don't think I'd build a rim with 20 or 24 spokes for CX. I would probably go with 32 with 3x lacing, and I'm a fair bit lighter than you (69kg), and have no qualms about beating the crap out of them. Rim and hub choice (and spoke choice) are all important considerations as well though. Also use brass nipples, not aluminum.
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