You should not have a whole lot of trouble with an alex or a macaw because neither are big time chewers (like cockatoos are, for example) but off the top of my head:
- no ceiling fans
- buffer zones in each exit to the outdoors
- covers for electrical wires and outlets
- non-toxic paint
- non-toxic plants (although if you are planning on allowing them to roam, I doubt you will be able to keep any plant in one piece
)
- stainless steel screens for the windows
- I would not put any expensive paintings, tapestries on the walls, chotchkies or books anywhere (I have two parrots that delight in throwing everything off the bookshelves shelves and chewing the books and magazines)
- get covers for the sofas and chairs (or they will end up with poop on them)
- keep a basket full of things they can chew (like old magazines, toys, pieces of wood, etc) handy
I would also suggest you keep a birdroom where they can sleep so you can put a humidifier in there. The reason for this is that they need high humidity, which people find uncomfortable and destroys your furniture, draperies, paintings, etc so having a room to themselves where they can have the environmental specs they need is always useful and healthy for them. AND, if you can have this room with an exit to an outdoor aviary... well, that would be fabulous for them, right?
I would also urge you to consider the fact that you are planning on getting a second bird of a completely different species to the one you now have... Parrots are flock birds and benefit immensely from having a companion of their own species. I know that people say that if you get another one and they don't bond is more work for you but even when they don't bond, they will get along and they will have somebirdy to speak to in their own language. I mean, if I was in an alien world, I would like to have another human to speak to even if I did not fall in love with him...