Featheredfluffkins
That sounds like a wonderful project and I'm sure your birds will love it.
I can't give any building advice, but when it comes to species I could make a few recommendations.
Gouldians and Owls should be fine (as long as you get them from someone who keeps his/her birds in an outdoor Aviary). If not, they will have to be acclimated slowly to the great outdoors; bringing them out for short periods of time, and slowly making these periods longer.
The same I would say goes for most birds really.
Birds who are kept indoors in constant controlled temperatures of say around 22C--70F, might need a slow introduction to fluctuating temperatures, sun, wind, rain, warmer temps, colder temps. etc.
Are you planning on an enclosed, or partially enclose area where they can shelter from bad weather? I would recommend an area like that.
Now to different species...
A male Canary, would add both colour and song to any collection, and one male would be quite content on his own.
Javas are lovely looking birds and come in different colour mutations.
Zebras, come in many different colour mutations, and as long as you don't get a large flock, they should be ok in a large aviary. In greater numbers, they can become a bit.... shall we say overwhelming, lol.
Stars, are colourful and peaceful.
One thing I am wondering is, once Winter comes, do you have indoor space for these newcommers?