
So back to the problem in hand –
I have been graciously told that the WM 14 machines are fine on a 500 bench. The metal drawers are actually 18" 460mm deep, but I think I can steal a little room.
The VM16 is a bit bigger and needs at least 500, plus sticks out a good bit, so perhaps it is the practical limit of this bench design.
The Axminster SX2.7 is illustrated mounted flush on a 515mm table top, so I suspect that it, and its SIEG brothers need a bit more space than I have. They are 30mm deeper than the VM16 and probably would need plan B – to take the space used by the Drummond – unless I do something special.
SX3 variants seem to have the same base as the SX2.7, and the VM 18 is getting big too.
The Drummond is no micro- lathe, it is about 42" 1070mm long, so takes up a lot of bench. However it is not going to be working soon, so perhaps its best place is on the back of the bench stripped down for restoration. 
That would leave me with a space of 1.4m, 55" between the South Bend and the door for a second bench on which I could put a milling machine. So I haven't made a decision at all.

Actually I have – I will build this bench around 490-500 deep, checking the crush gap between the bench and door. Whichever milling machine I get will go where it fits.
So on the basis that I don't want to have to upgrade in the future, I want good quality, I want to buy new, rather than fix up an old machine, I will be keeping to the smaller end of the model engineering, 3 1/2" gauge rail and probably 2" or smaller 3" scale traction engines, what machine should I get?? 
Thanks for all of your advice.