I have run the exact same encounter, but mine was with dire lions, and I gave them stat boosts as others died instead of levels and they also healed up as others died.
The first few rounds are a walk in the park if you party has fireballs and the like. They are just resource wasters and they alert the party to what's going on. As it goes on it will get a little tougher.
I found the key in this scenario is to make sure the guys can do DAMAGE!! You want the party fretting about whether to heal their buddy or to keep blasting. If they are just unloading their spells, this encounter will be over very quickly. Further, if the party is smart wall spells can dominate in this situation. If the horde is only able to approach the party from one side it will be a cakewalk.
You don't need gear, but you need a big bump to hitpoints, AC, attacks, damage, and saves as the forces get stronger. Else your CR 20 guy will be complete chump.
In general, my party didn't find this encounter too deadly when I ran it, but it did exhaust a lot of their resources. I will warn you that using a horde of mundane creatures like this is really a hit or miss proposition at high levels. If the party has the right spells, this won't be a challenge at all. If the party doesn't have certain key powers, it could be an intense fight.