36 spokes for sure. This is why: it gives you extra insurance to be able to continue to ride even if you break a spoke, allowing you to fix it later at camp or in the next town, rather than call for help.

36 spoke wheel does not mean heavy btw. We used to race on them. Heavy or light is a separate decision. I'd think frankly at 180lb total, you could ride something pretty light and be just fine. BITD I did long tours on 36h sew up wheels with ordinary 350g training rims. With clinchers the equivalent would be about 100g more. Perhaps a Sun M13 is a good choice, or maybe go up to CR18 for a bit of extra insurance. I would choose 28c tires if riding on asphalt, go to 32 if you will be mixing in some gravel roads.

Butted spokes make a more durable wheel, but they aren't strictly necessary.

I suggest you don't buy used wheels unless you are a pro mechanic. Rebuild yours at your LBS, or if you are on a budget, get a pair from velomine. I couldn't build my own wheels for what they sell a new pair for.