I agree with Richtor above, 100% choose a 18” over a 20” wheel, smoother ride, more sidewall, and better offroad capabilities. I also think it looks a million times better.
Nothing wrong with ordering wheels/tires online. I did that with my last F250.
That being said, if there’s a discount tire nearby, I’d really give them a chance. In my experience they have matched any other retailers, including those online. I called them up Friday to order my 17” Methods and 37” Toyo’s, told them the best price I found online, they looked it up to confirm, then matched it.
Also, most dealers charge $25-$30 to rotate tires. When you get your tires at discount, they do it for free. Not a ton of money, but over the life of the tires that’s around $300 if you rotate them every 5k miles.