How fun to be getting your horse! Congrats!
As far as the hay situation, what kind of hay is he used to? Simple field grass hay is most common here in the US, but alfalfa is pretty common as well (I think
in Australia alfalfa is commonly called legume hay, or something similar, but I could be getting my terms crossed between countries...). Grass hay tends to be the safer of the two, alfalfa/legume has lots more protein and tends to be "hotter." There are other posters far more capable than I to give specifics. Depending on what his new diet will be, I would call around and get around 10 bales of whatever variety he's used to, so you can switch him to the new kind. If he's used to the same kind you plan on feeding him, just watch him close.
Is there any particular reason you can't purchase a few bales from the same person who's selling the horse?
With the pasture, I would start out hand grazing him (but then I pretty much have time...) for half hour periods, and slowly extend the time. The reason I say hand graze instead of turnout is that some horses refuse to be caught after so short of a run. Just slowly work him up to the time he's going to be out.
If you must turn him loose, a grazing muzzle may be a good idea until he gets weaned onto the pasture. They aren't just for fat ponies, they're for any high
risk feeder, whether the horse is confirmed laminitic, insulin resistant, overweight, or being weaned onto a different pasture, or even in the spring when new, rich growth is coming up. If you'll probably never use it again, I'd even say buy a relatively cheap one and cut the hole in the end a little bigger each week until it isn't restricting his intake any more, then take the muzzle off. I would wean him onto the pasture, keep hay available to him when he isn't turned out, and keep a salt/mineral block avaliable. Other than that, a healthy yearling on maintenance really shouldn't need any other feed. Maybe if he drops weight in the winter on free choice roughage offer a feeding of a good "junior" feed.
As for the trailering, I honestly wouldn't tie him up. If you would feel more comfortable with him somehow secured, the most I would use is either a Blocker tie ring or The Clip in the trailer. I can't recommend these things enough; I use them myself for mature horses that are able to tie solid all the time, for peace of mind. If he fights, spooks, or falls, he won't hang up. The pic you posted is a decent quick release tie, but you have to be present to pull the snap and get him loose; something you won't be readily able to do on the highway between his old home and his new one.
I hope that answered some of your questions, good luck!