Get rid of theis Rohirrim idea imo, it won't get you far. The Rohirrim aesthetics looks a bit like Sturgia or Battania but these factions have the worst cav.
Javelin cavalry is very bad, it is a problem of tempo, because when you order your cav to charge you want them to do it ASAP, all together, in a mass. The jav cav will instead skirmish a bit and then charge 1 by 1 and die. The only way I would see them useful is with a lot of micro, having ONLY jav cav, not mixed with bow cavalry or shock cavalry and 1) have them attack the rear of enemy and send their hundreds of javs, 2) charge all together.
For that I would go with Sturgian they have 2 jav cav in their roster instead of just 1 in the Battanian. The desert mercenaries also have some cool jav cav or jav camels.
You get cheap horses and warhorses in Khuzait territory, the southern most and the northern most area. Then you use the battle loot.