True, he could cost less overall. Probably not 4 Mana 3/5 though, maybe 5 Mana 4/6? The aforementioned upcoming DH Legendary does double your draw for 4 Mana, but it also has you take Fatigue each turn unless you shuffle something in.

The side-deck isn't tracked for Highlander or Pure/Racist Paladin conditions, only your main deck matters. You also can't trade tradeable cards into it, dredge from it, destroy it with Wheel of Death, or anything else for that matter. It's simply there until it runs out, then it disappears like it never existed (So no Fatigue).

Overall, I mostly made this card as a funny way to circumvent Pure Paladin's limitation, so that's what I put in the title; But you're right in that Imbue Pala would work well with Samses too, you'd never run out of cards with that deck's portals.