The XIII Legion Ultramarines were the first legion I collected specifically to play Age of Darkness (The Horus Heresy). Originally I started them in 2012 to be my way back into Warhammer 40,000, it was around this time that Forgeworld had started releasing alternative armour kits which were heresy era in all but name at the time. I never really got beyond a few tester models for the palette and had more or less retired the project when the Betrayal at Calth boxed set was released and the 30K scene really exploded. With Mark IV legion astartes, Cataphractii terminators and Contemptor dreadnoughts readily available as plastic kits I dusted off the testers I did five years earlier and set about forging them into an army.
TACTICAL SQUADS ARE THE CORE. LOTS OF TACTICAL SQUADS.
Some gamers first experience of the Age of Darkness ruleset was through the Calth boxed set, me included and that meant infantry, lots and lots of infantry. Eventually coined a 'Centurion' list which meant infantry and walkers only this was a great way for me to start collecting. Add a few breachers in Mark III and I was ready for a few Zone Mortalis style games as well. The other reason for getting the infantry out of the way early on was because it's not often perceived as the fun part of building an army with lots of construction and large blocks of identical painting. Simply put it's a good idea to get the infantry out of the way first so you can concentrate on the fun stuff later. Unless of course you prefer painting infantry, in which case lucky you. Sadly I've seen too many projects come to a premature demise due to the rank and file being left until the end when painting fatigue is at its worst and newer shinier armies an unwelcome intrusion.