Terry front wheel (ISO 520 vs 540), big difference?
Interesting—Sheldon Brown's tire-sizing chart refers to the 540 size as "British juvenile or wheelchair"

In any case: The 520 wheel (which his chart lists as the Terry size—looking at the Terry website, I see they sell both sizes) will pitch you forward slightly and give you a slightly steeper effective head-tube angle. I don't think that would be a big difference. And you'll need to adjust your brake pads. In theory the size difference could be offset by using a fatter tire, but in practice, I think fatter tires are not made for the 520 size.

I've never heard of anyone using a wheelchair rim on a bike—the only possible downside I would imagine might be A) if they aren't available in the spoke count you want, or B) the braking surface. 

You might contact Terry directly and see what they think.