I'm thinking of doing about the same thing...
I'm wondering if a 17 inch rim wouldn't be better

Tire selection seems a bit better and you won't have to worry as much about the swingarm so much, as the tire won't be too much bigger than stock.
I'd like to get a cheap used rim if I can. I'm wondering... I'm guessing you may be able to mount an old GS550 wire wheel right on there. I think they're 18 x 2.15. I'm a bit shy of the 2.15 because it's harder to find dual sport tires to fit it. 120/90-17 is one of the best sizes to rim for when it comes to DS tire selection. 2.50 would be great size.
Can't say you'll find a suzuki rim that matches, though, unless you get a DR rim or something, and that's gonna be expensive. So far, I seem to recall old Goldwing rims are the right size... Don't know if the spoke specs are right, though...