Ah yes, my bad. Most of the monstrous mounts I've dealt with have always had the same T as the rider anyway, so it never really came up. Also good catch on the 3+. I forgot about the helm.
Anyway, if they issue a challenge with the champ, it's not always a loss on your part. Most champs only have 1 wound and 2 attacks. You can get a +4 if things go well (charge +3 over kill), and this kind of character isn't one that's going to be flying into a horde all alone. Careful planning could net you something nice. For example, if you double charge and flank him with a few trees/Wild Riders, all of the sudden they're disrupted and don't have any ranks. Also, for this set up, a Sword of Anti-heroes is another option for the same points. It depends on if you want that killing blow or another extra attack. You can make a pretty decent alter hero with that set up, actually. Anti-heroes, Helm of the Hunt, Armor/Shield, and Alter would give you 6 S4 attacks on the charge if you plow him into a character, but alters are hard to protect, especially with only 2 wounds.
It's a shame Lords are taken up by Wizards because an assassin highborn is pretty dangerous. The Spirit Sword is a gamble, but it is very powerful. However, it's over 50 points, so only a lord can take it. A Lord with Spirit Sword, Netlings, Helm of the Hunt, Light Armor and a Shield mounted on an eagle is 301 points, which generally means you don't have a high level caster.