An LSD is a must really and ratios will have to been chosen to suite the type of use the car will have.

If you are going for a 5 Spd gearbox you will need the RS2000 prop that comes as 1 or 2 piece.  In order for this to fit an english axle you will need the larger diff flange found on some later Escorts (RS2000).  They can be bought from most RWD Escort specialists and are simple to swap over.

If you are using an Atlas axle from a donar car you will have to alter the suspension mounts (it doesn't matter if you are going for a 4 link) and alter the diff angle, most people reccomend 3 degrees.  This is the angle between the rear of the diff and a line at right anglesfrom the floor.

I am using a 2.8 Capri Special LSD (Atlas) and will be sticking with it's standard ratio of 3.09 to save costs.  Once I can free up some cash I will get the diff serviced and changed to 3.9 using the crown wheel from a 1.6 Capri.

You can not use the Mk2/3 Capri diff on a Mk1 unless you have fitted bubble arches, even then certain offsets of wheels will rub!   The money you save fitting a 2nd hand diff should pay for some nice new wheels though.