Some comments on the above.
1. The exposure does not have to be fixed over the frames. However, preparing the frames for stitching can be *very* laborous. One should start out woth raw data and "equalize" the frames in raw conversion. Sometimes graduated transitions may be required, even more work for preparation.
Equalizing the frames by the blending is an abuse of the process with usually horrendeous result, either as banding particularly on clear sky, or by ruined colors.
2. The camera does not have to be level. In fact, in many cases it *can not* be level. However, the *plane of swiweling* needs to be level in order to avoid having to clip large segments.
3. There is no need to shoot with fixed focusing. One can achieve a large DoF by shooting with a long lens (long relative to the scenery), so that nearby and far objects are in separate frames and refocusing every frame. However, refocusing changes the field of view; PTGui has to be notified about this (by unlinking the FoV).
4. The father of stitchers is Panorama Tools. This is a package without human interface; Hugin (free), Panorama Tools Assembler (cheap) and PTGui (expensive) are based on Panorama Tools. These can stitch virtually everything, what is stitchable.