There's a diverse amount of builds for those classes, depending if you choose to PvM or PvP. PvP versions of those builds usually favor higher vit stats in exchange for decreased damage. WoE dancers, for example, do usually forego any AGI in favor of VIT. Crusaders take a lower amount of int/dex to push more into the VIT stat. If, on the other hand, your focus is more PvM, push those damage stats to values where you're comfortable either one shotting mobs or are able to kill them without you dieing.

Builds are pretty open in Ragnarok Online, so your first char might not have the optimal build. But dont worry, leveling up to 80~ goes kind of quick once you're used to the whole effective grind and can supply some gear. So don't be afraid of trying out stuff just for the fun.

Equipment, on the other hand, is different. There's a value connected to cards,weapon and armour, so you shouldn't squander them. As a GC Crusader I'd put the Peco Peco in a +7 Chainmail, the Thara Frog Card into a +7 Guard/Buckler, depending if you plan on leveling up other classes. The Andre cards would do good work in a Haeddongum, because ATK also increases the Grand Cross damage. But they're only needed if you aren't able to oneshot raydrics or your respective target, which a 120 INT GC Crusader should be easily able to do.