Its not that difficult and I've had no probs with my Insurers - NFU - Just declared it as a 6 seater. Likewise no issues with MOT - the rear seats are "not part of the test" as far as my MOT station are concerned - and they sell Landrovers so they should know !
The key thing is to measure and mark the space where the windows are going very carefully. How the windows go in will depend on what you buy, most of the aftermarket windows are made to bolt or rivet in place, with the fasteners covered up a two piece rubber "filler". Basically you make up a template, mark it out then drill a hole large enough to get a jigsaw in and cut out the section ( same for main windows or alpine lights. Then you offer up the main windows, drill through where the fasteners are going ( Rivets are easist, last time I used m4 stainless countersunk machine-screws and shake-proof nuts. Take the window out again, silicone all round it, offer it up again, bolt/rivet it up and take a break. This is because putting the rubber filler in is a pain. The filler comes in two parts, a fatter big bit with a smaller triangular bit that squashes the edges out and tightens it all up. Washing up liquid is a great lubricant for this bit, as you attempt to push the triangle into place. A handily sized blunt screwdriver maybe ? The professionals have a nattty triangular wire on a handle thingy.
The alpine lights are similar, except they are held in place with just a rubber surround....and the triangular filler job again.
Here is mine .... ( The alpine lights were already in it but the sides I did myself..)
