Second point first. Flipping your axle: alot of axles have a bow to them so when the trailer is in the air the tires will be tilted (tops out and bottoms in)so when it has weight on it the axle flexes and the tires take a more vertical stance. If you flip the axle you may have some tire wear problems.
Tires. How much is your finished trailer going to weigh? Trailer tires are built to carry more weight than a passenger tire. Trailer tires also have stiffer sidewalls. They will also ride rougher. If you are under the capacity of a passenger tire, I would go that way. They will be less expensive, easier to get and more options.
Can you go with a taller tire? Do you have the wheelwell clearance? You could have some spring shackles made to lift it. You could have the springs re-arched or replace them, depending on their condition.
If you have shackles made, build them like this. (yours probably doesn't need the bend, just an example) The center spreader bar holds the side plates to keep them from scissoring.
