friend recommended .350 Legend and .450 Bushmaster. In my research I saw that Bear Creek Arsenal seems to have affordable prices. Any recommendations?
By get into wild boar, you mean feral hogs, or do you mean you plan to travel to Europe/Asia for actual wild boar hunting? I am going to assume the former since you are talking about taking an AR15.
You can hunt feral hogs with just about anything you want (some state laws may apply, here). You don't need something like a .350 Legend or .450 Bushmaster, although they are perfectly good for hunting feral hogs. What I am saying is that you can take feral hogs just fine with lesser calibers.
Will you be hunting at night with NV or thermal? If so, some/many calibers are apt to produce too much recoil and may shorten the life of your optics and/or void warranty.
Will you be sniping/meat hunting hogs or attempting to do feral hog control. If the latter, you may not want the higher recoiling calibers as they will tend to be comparatively slow on followup shots unless you are a super shooter or super powerful stud. For example, I have a buddy that shoots his AR10 in .308 the way that I shoot my 6.5 Grendel. The dude is a fireman and built like the kind of guy that could carry you, your wife, and your family St. Bernard out of a house fire in one trip.
I have hunted with a .50 Beowulf and while it was great, followup shots were not timely, although the caliber and ammo I was using did do impressively damaging things to hogs.
I would suggest going with a smaller caliber for less recoil and then going with a quality expanding bullet that will reliably expand fully or nearly fully in the distances that you will be shooting.
If you are just meat hunting, I would go with a decent bolt gun in whatever caliber you want and save a bunch of money on the rifle. I would still suggest going with a good expanding bullet.