As mentioned by ChaseC, situational damage taken by a specific target is impossible to predict due to the intervention of a variety of variables, such as buffs, debuffs and the target's armor stat. It should be fairly straightforward to figure out base damage, though.
For the sake of simplicity, I'll ignore the effects of Runes and Masteries in this answer. When talking about effects relating to the ability, it'll be the level 5 version, and champion base stats will be for level 18.
You've already quoted the wiki page in your question, so I won't repeat that here.
Each arrow fired in the flurry of 5 arrows has damage equal to 27% of Ashe's Attack Damage (AD), meaning the entire salvo has a Physical Damage (PD) output of 135% AD.
At level 18, Ashe's base AD is 100, therefore, we can calculate the PD dealt by Ashe before resistance reductions with a very simple calculation of:
100 AD x 135% = 135 PD for a single flurry.
Now lets consider this with items - a typical build for Ashe has a end-game build of Beserker's Greaves, Infinity Edge, Phantom Dancer, Last Whisper, The Bloodthirster and Banshee's Veil, with an Elixir of Wrath consumable to finish things off.
Altogether, these items provide additional (relevant) stats of:
- +75% attack speed (25 BG, 50 PD)
- +225 AD (80 IE, 40 LW, 80 BT, 25 EoW)
- +55% Critical Strike Chance (20 IE, 35 PD)
- +50% Critical Strike Damage (50 IE)
With an updated AD total of 325, the total PD output of Ranger's Focus after being fully built is:
325 AD x 135% = 438.75 PD per auto-attack while Ranger's Focus is active.
In addition, Ashe's passive Frost Shots guarantees critical strikes on a target with the debuff Frost, applied by auto-attacks and certain abilities. Given the nature of Ranger's Focus, the target is more than likely already affected by Frost, giving the increased damage of:
110 + (Critical Strike Chance % × (1 + Bonus Critical Strike Damage)) % AD
If we input the appropriate numbers from the improved stats above, we get a calculation of:
110 + (55% × (150%)) = 192.5% AD
Assuming the target is already debuffed by frost, the damage taken (before armor reductions) is:
438.75 PD x 192.5% = 844.594 PD for a single flurry of 5 arrows.
While we're here we may as well go deeper. Ashe has an attack speed of 1.224 at level 18. Including the items above, a late-game Ashe will have an attack speed of 1.974. Upon activation, Ranger's Focus grants a further 40% attack speed, bringing the total to 2.374.
Once triggered, the ability lasts for 4 seconds. Assuming the very first flurry is right as the ability is activated, Ashe will be able to send out:
4 seconds x 2.374 attack speed = 9.496, or 9 auto-attacks over the duration.
If we apply this to our original equation, we'll find the maximum damage achievable over the 4 seconds Ranger's Focus is active, assuming we're relying solely on auto-attacks:
438.75 PD x 192.5% x 9 instances of the ability = 7,601.344 PD, which is the maximum possible damage output of a perfectly timed Ranger's Focus (with this build). This comes out with a possible (physical) Damage Per Second (DPS) of 1900.336
This is correct as of Patch 5.11.
As for useful items to benefit from the on-attack benefits, anything with lifesteal is a bonus. If you prefer to have a more attack speed focused build, Runaan's Hurricane could be a useful pick, as it's unique passive activates for each arrow in the flurry.