Personal Union Advise please

I am in an ironman game and I haven't ever done much personal union stuff before (except earlier in this game when I fell victim after thrice having the heir of the same king killed in a hunting accident). Now I have the opportunity to try to PU Austria, who has been my biggest ally all game, but I don't know how this all works out. Will I go to war immediately? What happens after that? As a powerful country, will they have a high independence desire?

I have a truce with pretty much the whole world because I just got done beating down a coalition of, well, everyone in Europe. So I could go to war with Austria if necessary, but I don't know what will happen down the road. Is this a good idea? Will they stay PU'd or will I get an opportunity to straight up annex them at some point after the PU?

http://imgur.com/a/uhhvz