You may want to have the axle assessed by a professional. If the axle was flipped to give more height, that spacer you mentioned may have been an attempt to correct the negative camper that came as a result of flipping an axle that was fine. You can't just flip an axle willy-nilly. If the axle is not bent, the positive camber built in at the factory becomes negative camber, resulting in abnormal tire wear.
If the axle is actually OK, you can have it flipped back over and replace the leaf springs. Then, if you want, you can have the axle mounted to give more lift.
The trailer shop I used to work with flipped straight axles that had been bent from time to time, but it was usually on light
utility and boat trailers that weren't used for long hauls. The problem with doing this is that axles don't necessarily bent just right and you can be changing one problem for another. Also, some shops are set up the bend axles, like they used to do on the old I-beam front ends back before independent front suspensions.
If it was my trailer and the shop you go to says the axle is indeed bad, you can likely save money by just buying a new axle outright, rather than incurring all the labor involved.
and wheels are OK if you have the room for them under the fenders.
Don't let them talk you into a heavier axle. Go with one rated for your trailer(2000 lb?). Otherwise that baby is going to be all over the road and you are going to have to put everything back on the shelves at your destination. There have been several mentions of this in a couple of threads lately.