I would add the following to the list of useful references;
W Y Carman Indian Army Uniforms( 2 Volumes - Cavalry & Infantry).
Osprey Men At Arms - The Gurkha Rifles
Osprey Men At Arms - North-West Frontier 1837-1947
Osprey Elite - The Gurkhas
National Army Museum- Soldiers of the Raj The Indian Army 1600-1947 by Alan J Guy & Peter B Boyden
Blandford - The North West Frontier , A Pictorial History 1839-1947 by Michael Barthorp
David & Charles - The Indian Army by T.A. Heathcote(not a rich source of uniform detail but has several tables for that period showing Class composition of the regiments).
http://www.members.dca.net/fbl/ - a website devoted to old photos of the British and Indian Armies, not been updated very recently and was never fully expanded, gives some interesting links to other sites.
One of the Military Modelling "Annuals" they produced yearly in the early eighties had an article on the 2nd Afghan War with about a dozen coloured drawings( might have been Scollin's work), think it was the 1982 annual.
A C Lovett's colour plates that used to be available as postcards from the NAM although for a slightly later period give a good indication of the varities of turbans worn by the different classes