Parallax is a much larger problem when shooting at close distances so you will absolutely need a panoramic tripod head that allows you to pivot about the lens centre of projection, and be very careful with the setup.

You can use any lens, though the longer the focal length the more images you will need to shoot and stitch to get the coverage. If you're planning to view the results on screen then just use your widest lens (18mm). If you're going to print large you should consider using the 50mm.

With regards to stitching images, lens distortion in individual images can be corrected in software with an appropriate profile, but any panorama you generate automatically is going to bend straight lines. There is no way to have a 180 degree field of view onto a flat image whilst maintaining straight lines in the general case, it's a mathematical impossibility.

However, if your green house walls form a rectangle, you could produce a pair of rectilinear 90 degree panoramas, and then join them along one of the vertical edges, to produce a pseudo-rectilinear 180 degree panorama. You may have abrupt angular changes in the floor or ceiling with this approach. This can be done manually in photoshop, although there are tools to generate rectilinear panoramas, for example PTStitcherNG. A tutorial on how to use this tool to straighten a cylindrical panorama can be found here.