It is difficult to answer your question without some details...
How much Stardust do you have?
How much Rhyhorn candy do you have?
Does Rhyhorn spawn frequently where you play?
IV really doesn't make a difference in combat because damage values are rounded very aggressively. Unless you are exclusively collecting high IV Pokemon, I would ignore IV.
Only you will be able to make this decision. People who answer your questions will give you subjective answers based on their situation and play style.
Personally, I'd wait until you hit level 25-30 to begin powering up your Pokemon. You WILL catch a higher CP Rhyhorn... trust me. This will save you a ton of dust. IV is TBD by your RNG.
Day 1 Trainer. Level 32.