I think that Seer is powerful (not too powerful, but powerful enough) at $2. Also, I personally prefer the card without the Attack subtype (Possession also lets you take a look at an opponent's hand and isn't an Attack, and I think it's more interesting if it can't be blocked with Moat/Lighthouse - though, I can see why you would want to make this trigger Horse Traders' reaction), but that's a matter of opinion.
I actually have been wishing that this card existed in official Dominion, and I think that the uses for this card aren't as niche as they might seem. First of all, it's always useful in end game tactics, and with Possession, deck manipulation attacks, Council Room, Soothsayer and Governor, Masquerade, Bishop, against Envoy and Advisor, against an incoming Cultist chain when you have $3 and a Watchtower in hand, etc it's even more useful.