When I replaced my wheels I was thinking of replacing the tires because of everything I have read about tires made in China coming apart. I have Constancy tires on my 300tq. I googled the tire and could not find any reviews where their was a tire failure. That does not mean there is not one out there. I decided to keep the Constancy tires when I replaced the wheels. Maybe in the future when I have to replace tires due to normal wear I will probably buy something made in the USA.
As far as 18's I don't know might be pushing it plus we really don't need to be making the trailer any TALLER. I think you would be OK with 16's.
Tires come apart for more reasons than just the fact their made in China. I check tire pressure and wheel torque before setting out. I also carry a hand held infared temperature gun. A quick zap on each wheel and tire when you come to a stop and do your walk around will tell you if you have a low tire or brake or wheel bearing problem.
I learned alot about trailer wheels when shopping. When it comes to trailer wheels and automotive wheels they are different. Trailer wheels carry more weight and have a 0 offset. hard to find a automotive wheels with a 0 offset.
I got my wheels from E Trailer.. They were a good price plus they had free shipping. Most online wheel suppliers wanted $20 to $25 per wheel for shipping You will also need to figure in lug nuts and center caps with your new wheels.
Curious on what warranty work your needing? I have been pretty lucky so far any small problems I have been able to fix myself.
Before and After