Doors

I want to replace my defender doors with series doors as almost everything but the skin has rusted away.
I assume they will fit?
What is the difference between the series 2/2a and series 3 door tops as they seem to have different part numbers - is it just the window catch?

You will probably have to change the receivers on the door pillar.
I think you are right about door tops, I'm not sure you can even get Ser II door tops now.
Ser II doors have a different check strap arrangement

like dave says, you'll av to change the striker plate since you'll be going from anti-burst to the slam type arrangement (i.e, the ones that open on their own when you go over a big bump) IIRC you have to drill new holes in the pillar as they don't line up. Everything else should fit fine