id suggest 21/15 strenght build. but the same build you would use as regular light cav, you should use as heavy cav.
Tale note though that heavy horses have been quite nerfed, and altough you can still survive descent ammount of arrows, headshots would make your horse die in just 2-3 hits.
Your horse will die more quickly from from melee hits then before, so no longer can you walk away once being stoped in melee clusterfuck.
plus the repairs are extremy high (4k gold) and occur often, so be sure you have some cushon money saved up, cause you will loose your gold quickly.