How do I attach a ledger to a stucco wall?

Most of the pre-engineered aluminum patio roofs are of "aluminum wrap with foam sandwiched inbetween" for some R-value. The are typically "let in" to a receiving channel equal to the thinckness of the roof panels themselves. Without knowing your elevation, I will assume you can attach the channel to the house as you suggest, and if you do that so that it pitches just below the fascia board with near -0- clearance, you can use a piece of angle flashing to counter flash by slipping it under the drip edge that covers the fascia board. If you are attaching it to a gable end, than you can first make a shallow saw cut into the stucco, and insert a "Z" flashing in that, and caulk it in with Vulkem Caulk.

Stucco is just concrete used as a finish. You need to find a solid support inside the wall. Drill small holes where the railing will attach to the house. You will know you have enough support if the drill bit is being resisted to the entire length of the bit. Place a piece of wood at that point to attach the railing to.

With the ledger board on a flat plain you can get away with a good latex caulk. If you don't feel that would do it, you can cut a slot just above the ledger and inset the flashing into the wall and bend it over the ledger, but you will still have to caulk the slot where the flashing went in.