I love off meta picks but there are some questionable choices. The way I think Ashe support works is to abuse her innate slows and poke, her utility.

You want as much crit because it triples her slow and attack speed so you can reapply the slow. In lane you will be able to poke and harass a lot because your auto attack range is 600 against most marksmen with 550. AA them when they try to last-hit.

Rylai's and frozen mallet are bad. Every damaging spell of her and her auto attacks slow!!!! Also in bot you need armor and magic resist. Most supports have 15 armor in their rune page.

Ok, here is what I think can work.
3x quints of critical damage
9x marks of attack speed
9x seals of armor
9x glyphs of magic resist

Spellthief->Sightstone->Runaan's->Eye of the Watcher's->Lucidity Boots->Black Cleaver->Zeke's Harbinger/Wit's end->Support Item
The build is cheaper than yours which is important for a low income role. If you do really great then get IE.

Fresh Blood and Natural Talent instead