I haven't found a 15" ST tire that will last much past 3 years. In their 4th year I blew 3 in 2 trips, the tread looks fantastic, even after coming through the trailer floor. They separate and break down. With a 10 year old trailer I have gone through 4 sets of ST tires.
16" Rims will run you $70/ea for an exact match to the WW 15" design (at least the one they had in 2006). If you are seriously strapped for cash buy the rims ahead of time. I bought 7 in the spring.
I bought a set of the Bridgestone Duravis R-250's over this last weekend. I went with the larger 235/85 R16 - 10mm wider is no problem but the jump in diameter took some triple measurement to feel comfortable. When in doubt, get out the measuring tape. I have 7" between tires right now and plenty of vertical clearance because of a previous axle flip. If you don't have that much room use one of the smaller sets. All diameter and width measurements are on the tire MFR sites, it's best to not assume they are the same as your existing tires.
Mine will be shipped, mounted and installed sometime in the next 2 weeks, both Costco and Walmart get them are really good prices (about $50 less than retail).
Good luck!