If you are planning to shoot real estate sphericals with a fisheye lens to speed up your photography then I'd recommend that you use a manual pano head rather than the Panogear.
It will be quicker and easier to set up and shoot with a manual pano head and the nadir shot (if you wish to make one) will be easier to shoot and the nadir foot print of the pano head will be smaller.
Robotic pano heads really come into their own when you are shooting medium-high res.spherical, or more typically partial, panos with longer focal length rectilinear lenses.
The T&C controller will cost Euros 299 inc. VAT; a Nodal Ninja 5L (factory irregular) manual pano head will cost approx Euros 368 inc. VAT.http://www.red-door.co.uk/pages/productpages/nodal-ninja-panorama-head-NN5L-FI.html
Of the lenses you mention I would not recommend the Sigma 4.5mm FE - that one produces a circular fisheye image and does not fully utilise the sensor area.
You could also consider the relatively inexpensive but fully manual Samyang 8mm fisheye (and the many alternatively branded clones), the Sigma 10mm fisheye or the Tokina 10-17mm zoom fisheye.
Will your real estate clients wish to display invidual panos or virtual tours fullscreen and expect to be able to zoom in; or will they never be displayed fullscreen?
The resolution of spherical pano image required will determine which focal length fisheye you will need to use on your Canon 500D.