Hi Sandie
The army records for WW1 are the pension files (in WO363 at Kew) and the service records (in WO364 at Kew). Both are on microfilm. The pension files were obtained from the Pension Department to try and fill the gap left when 70-80% of service records were destroyed during WW2. The service records are the 20-30% that survived. You should check both as it is possible that a man has a record in each series. Equally if you do not find a pension file for a man who you know received one, you should still check the service records, as if his papers were not destroyed they did not need to obtain a copy from the pensions department.
Ancestry were awarded the contract to digitize the WO363 and 364 microfilms. They have completed WO363 and have completed up to the Ns in WO364. So the answer to your question is that only Ancestry can provided anything online. BTW..the definition of 'WW1' is anyone discharged between 1914 and 1920 inclusive. If he served into 1921 (as an OR) his papers should still be with the MOD.
Regarding NoK details, you will not find these on the medal index cards, except in a tiny %age of cases when an address appears on the back of the card. Only the cards being published by Ancestry show the back.
I have only come across NoK details in a few places. Firstly on the service records. Also on SDGW and the CWGC website, obviously only for men who were killed, and on the National Rolls. These were produced by subscription - either paid for by the family of a man who died, or the man himself, as they also record men who took part, without being killed.
If the man was alive in 1918, and over 21, you could ask at his local council office to see if he appears in the Electoral Roll. Either in the main roll, or (better) in the Absent Voters List which usually give full army details of regiment and number. You can then link the address to the rest of his family. Not all councils produced an AVL.
The chances of finding a service record is greatly inceased if the man was in the Guards or Household Cavalry, as they held a second copy in a separate place. Or if the man was commissioned during WW1 as a copy of his OR service was attached to his application, and these were not destroyed during WW2.
If you post the man's details I am sure someone with an Ancestry account will do a lookup for you.
Ken