Yeah you don't want a straight axle and
sprung over. I mean you can do what you want, but that would not only be overkill, it would make the trailer less stable, really hard to get into, and cause way more wind drag. It's already going to sit pretty high up above your truck.
I went with a straight axle and springs below the axle. What I've found is that if I need more height than that to get down a road...I don't want to be going down that road.
The cost for a new axle and installation was pretty reasonable. I just did it last spring. Just make sure to use a trailer shop, not an RV shop.
Mine was also cheaper because even though I didn't want to, I kept the old springs. For whatever reason, the guy at the shop didn't want to weld my frame. It's a pretty thin frame and I think he just didn't want to mess with it. So I've got a new axle but the original springs.