It's not difficult but a hand would be useful to apply gentle pressure to keep it in place. I used two clamps and some wood by myself. Just make sure you have the rubber seal the right way around.
If you slacken the pump pulley bolts the belt should move enough to align every thing, it can give almost one belt tooth variation so you have to asses whether you need to move the belt one tooth on the pulley to get the correct alignment.
Doesn't sound too bad, There is both a spacer and a strap on the inside of the windscreen bracket anyway as the spacer allows the bracket to clear the lip on the top of the bulkhead. A 6mm gap is well within tolerance. I have looked at some new units on line wich seem to be good value.
I bought an Evolution metal chop saw from Aldi many years ago. It has cut everything I have thrown at it. As it is only a mitre saw and not a compound mitre you move the material clamp and not the blade angle which is the most annoying thing about it. but it has earnt the £90 it cost me back then.
Welcome to the forum. There are a number of suppliers for the front doors, start on Ebay. as for the rear door hopefully some one else may know a good supplier. Good luck with the conversion.
The Defender was the same, hand built on a production line with pre-made parts. Does Mercedese do it better? Morgams and most kit cars use pre-formed panels eithr pressed or moulded.