I don't recommend a K&N air filter, those things allow soooo many dirt particles into your engine it isn't funny. Stick to a regular paper OEM air filter box, they don't provide any additional HP, that's for sure, but they're the best at keeping the engine from inhaling dirt.
There are not too many affordable options for increase in HP in these Rodeo's, main thing for optimal power is keeping them stock and in-tune.