The cheapest route that will give you all of the rules you need is the core army list:
https://www.forgeworld.co.uk/en-US/The-Horus-heresy-legiones-Astartes-Age-of-Darkness-Army-List
You also need the rulebook which gives you the unique rules for the Legion. Here you can turn to the other red book (
https://www.forgeworld.co.uk/en-US/The-Horus-Heresy-Legiones-Astartes-Age-of-Darkness-Legions), which is cheaper and gives you the lowdown on all the other Legions. Or you can get Book V, which has the Word Bearer fluff/organization/rules, in addition to the same treatment for Ultramarines, the rules for Militia armies, and the Calth campaign setting.
Book
VI has some stuff not available anywhere else, but nothing too relevant for Word Bearers unless you decide to run Shattered Legions or Blackshields. The only stand-out thing is the Delegatus is a nice cheap
HQ to unlock Rites of War, but I think the generic delegatus rules are available from
FW's site under Autilon Skorr.