He rides a WR400 now, Adam. And he's 5'8", 200 lbs.
No way, a 450 is not the bike to get. First of all, you are likely too small for one and you wont be able to throw it around easy enough,
Riding with no fear is one thing, riding with no sense is another. The YZ450 is capable of generating very high speeds in an astoundingly cavalier manner, and you can find yourself in way over your head before you know it.
If you're used to the size and weight of the 400, you can probably handle an '05 or later model. The '03 and '04 have too much of a "muscle car" sort of power delivery, and that can intimidate novice riders to the detriment of their learning experience. The later models feed the power in a much more manageable fashion.
As far as two strokes being good learning platforms, I don't agree, necessarily. My son learned more in the first two months on a 250F than in the previous two years one a CR80.