The Nikon D3100 replaces the D3000 as Nikon's entry level DSLR (Bottom of the range).

It's major disadvantage is that it doesn't have (AEB) Auto Exposure Bracketing.

The D5100 the replacement for the D5000 does have bracketing albeit limited when compared to higher Nikon models. The D5100 (AEB) is limited to 2ev 3 frames the same as the D5000 the model before it.

Once you jump up to the D7000 you get (AEB) 5 3 frames. The Semi Pro Nikon versions like the D300, D300s are 5 far superior with 2, 3, 5, 7 frames.

For HDR work (AEB) is a requirement and most desirable.

Based on what I have read in this thread about your requirements your best and cheapest entry will be the Nikon D5000 (if you can find one) or the D5100 with a Samyang 8mm Fisheye. The Samyang is sold under many different names, some are Bower, Rokinon, Falcon, Polar, Optics Pro, Vivitar, among others. All are the exact same lens optically. The Vivitar differs in appearance somewhat but is still the same Samyang lens.

This combination is well suited to the Nodal Ninja 3 panohead or Agnos MrotatorU. Panoheads above this are an overkill to your needs. Panoheads under these are less than recommended.

Regards, Smooth   

It is also worth mentioning that the Sigma 8mm and Samyang 8mm are NOT the same type of lens.

The Sigma comes under a "drum" type lens and will require a minimum of 4 rotational shots.

The Samyang is a Stereographic type projection, more like a "fullframe" and will require 4 rotational shots plus a separate Zenith/Up shot.

You will get a slightly larger maximum panorama image using the Samyang but this lens is totally manual and does NOT communicate with the camera. Not that this is an issue if you understand cameras/lenses and photography, as all your settings should be manual anyway.

It is a big step up from the Coolpix to DSLR and will be quite a learning curve.

Regards, Smooth

The Nikkor 10.5mm Fisheye will attach and work with the Nikon D40 no problem. The limitation are with the camera itself. You certainly can use this combination to learn and progress.

You can move to the D7000 and use the same Nikkor 10.5mm Fisheye lens with all the benefits of the better camera body.

Understand though, the Nikkor 10.5mm Fisheye will require you shoot 6 rotational shots + 1x Zenith/Up shot and 1x Nadir/Down shot (if desired). Making it 6+2 in fullframe stitching mode.

The best shooting pattern with this set up is 6x rotational shots with -10 degrees tilt down. 1x Zenith shot at +65/+70 degrees tilt up and then the offset Nadir/Down shot.

You also can use the Samyang on either.

The shooting procedure will be 4x rotational shots at -5 degrees tilt down, + 1x Zenith/Up shot at +70 degrees tilt up and 1x offset Nadir shot to cover the tripod position. Making it 4+2 in Fullframe stitching mode.

Regards, Smooth