Unless you are going to do this yourself, I would just go to a local
contractor and start there with him.

All steel doors are not the same, nor are their cores. The same with
fiberglass. The skins on both steel and fiberglass vary quite a bit
in thickness as do the composition of their cores and their respective
"R" ratings.

Ditto if you get prehung. With the actual product varying all across
the map, so do the prices. And since you will be relying on pricing
in your area as well as the exact manufacturer of the units sold
locally, it would be a bit of a stretch to make a qualified remark.

You should also consider your finishing skills. When I do a steel
entryway door, I use commercial alkyd paints mixed with different
thinners and a metallic hardener and sprayed in multiple coats. I
found no one wants to see brush strokes from using a can of Rustoleum
on a brand new door.

Fiberglass can present its own problems with adhesion, and with latex
finishes, no matter what the manufacturers say, I have always had
problems with adhesion in high traffic areas.

Staining fiberglass to look like wood.... take a class if they offer
it. Getting the color on isn't difficult, but getting it on evenly on
a semi slick surface is a trick. Then of course, you have to seal it.

Good luck.

Robert