Aesthetics aside, even battania scouts are fine cavalry. Yeah, cav X has 40 more armor and cav Y's sword does 8 more damage, but the skills are similar and you won't notice much of a difference when you're riding down archers and crashing into the backs of infantry. The biggest difference comes when they are dismounted.
Use whatever you like, they all get the job done. A tip for mid to late game is to capture bandits. The various different types upgrade into the noble line of the corresponding culture. With a big army you can get groups of 10 or more to instantly join your army.
As for the rest, in battanian country, one of the towns frequently has dozens of battania horses for 50-70 sense. The khuzait are a pretty reliable source of warhorses. I like javelin cav fine. I wouldn't go out of my way to get them as archers are better, but if you play sturgia they are good at distracting other cav and disrupting archers... and they're all you get until you can farm sea bandits for druzhinnik.