I think most determined weapons raise will and attack although the latter not by much. There's a unique weapon near the end, the orbis bellum which raises all of Rheinhart's major offensive stats (will, attack, agility) but that's somewhere near the end. Rheinhart has a talent which up to 50% of his agility can go to his ability DPS. You can try finding Fierce x of quickness or Swift x of Rage clothing and armor items instead. Determined x of rage stuff might show up but I haven't seen it yet.
Attack does go into ability DPS also but not by much I think. Will is really the stat which raises it. I don't think there will be a situation where you're direct damage abilities do less damage than your standard attack. In Rheinhart's case you should rely more on his abilities to do damage and just use his standard attack to refill his Focus.
That being said you're free to concentrate on a Rheinhart that relies more on standard attack with situational use of his abilities. He does have abilities that are useful for more than just DPSing i.e. Mirror Manx distracts, slowdown by his clockwork trap or by GoA, companion (and self) healing by creative destruction, etc.