I would as a buyer be looking for the most original Sandman as possible.

On my hit list would be:
a good colour (for me the bright 70's colours are the go)
308
m21
original engine, chasis, painted colour, interior, etc.
original documentation/history

Then try and stay as original as possible.

I have added power steering and want to add air con to ours in the future. As I do not have the original sales documentation thee "could" have been added from factory anyway.

Would never recomend painting in a different colour. Way to expensive to change later. Interior ok to change as this can be returned to stock pretty easily and cheaply. (we should have buckskin but went for black instead)

With more and more Sandmans coming on to the market nowdays it is not too hard to wait for the right one that will tick all the boxes.

I agree. Buy and build with the intention of selling even if it is not on the radar at the miniute. Therefore buy the car you want from the outset even if you have to wait to do it.