Another member here and I are building a mill as well. This is the second one I've built. On both I use 19" sheaves, (pulleys), fitted with very tight fitting belts. They're probably not quite as good as the solid steel wheels, but they've worked well for me. A lot of the major manufacturers use the same set up, Woodmizer for one. On this mill, we'll be using urethane belts instead of regular rubber as the sawdust won't stick to it.
Here's where you can find the sheaves:
Surplus Center - 18.75 O.D. H-BUSHING SINGLE GROOVE PULLEY
Look around their site. They also have 'H' bushings that you'll need to attach the sheaves to the shafts as well as the shafting, pillow block bearings and other items that will be useful.
Personally, I don't care for the tire set-ups. They seem to work well for most people, but when I sink a bunch of time and money into a mill build, I have a problem with the end product looking like something that was converted from a trailer. I don't think when all is said and done that the set up I use with the sheaves is any more expensive than playing around with tires. Blade tension is pretty important to blade performance and longevity too, and I don't see how that can be achieved with tires. While the bigger the bandwheel the better, (in terms of blade life), I haven't had any problems with the 19" size of the sheaves. Again, a lot of the major manufacturers use the same size as well.