In general, you'll want to follow the main storyline (and if you care about titles, make sure you do the sidequests that are hidden by default). Spending AP is fine for two reasons - you can get unlimited amounts of it by playing so all skills can be maxed out, and a skill reset is given in the growth reward for reaching level 90 in case you feel you made a mistake in distributing it.
If you want to create a character using the free slots, you would need to delete 14 characters (7 free slots are available on an account).
If you want to create a character by purchasing a premium slot, you would need to delete 1 character (premium slots can be added to an account to push it to at most 20 characters).
The title in the description is incorrect for level 5 abilities. You need the 50 kill "Dullahan Dominator" title for level 5 abilities. Lochlann Plains abilities will then work on Lochlann, Berbhe, and Donegal dungeons.
For armor and accessories, the primary gains are in Wil increases, which no longer matter to most endgame players (although the benefit can be seen on "fresh" characters with small title counts). For weapons, though, I feel it is still fairly significant. I see two common cases. In the first, most people do not surpass Dullahan's natural attack cap of 28500. In the second, it is not improbably for someone to have the 2000-3000 attack limit release that makes its attack cap (of 31550 at max non-event release) actually difficult to reach outside of soaps. However, these are limited cases in terms of the percentage of endgame content. I think one 28500, one 27000, then multiple 25000 are the caps we have access to, and most people can hit the 25000 caps.
105 crit is needed
28 comes from Critical Hit Mastery 3
52 comes from a max-synthed level 90 weapon
15 critical comes from 2000 Willpower
Meaning 10 critical is needed from any of these sources:
Stigma (6 crit) or Master (5 crit)
Subdued (2 crit) or Petite (1 crit)
Immoral (8 crit), Righteous (2 crit), Divine Punishment/Maelstrom (2 crit), Spirited (6 crit), Decisive (3 crit), etc.
Ein Lacher's achievements (3 crit)
Sea of Reflection achievements (5 crit)
Most armor prefixes/suffixes give 1 crit (Reinforced, Expeditonary, Sentinel, Well-balanced).
Hence, if you honestly can't get enough base stats and slap on at least Well-Balanced Sentinel plus a Master on most of your gear, you probably should be locked out of the battle. 105- crit has 3% chance to crit Dullahan.