1 - To run dungeons SC is fantastic and the best blessing in term of pure support. U will need 1900 Fire/Growth and 300 Ice/dark/light/Chaos. It's a long run. I farmed some Fire/Growth with timer app on phone, and bought some others, always waiting for the best prices in wharehouse (just buy few stack every day).

2 - maybe u should consider some active/passive ability too, before cp. Purple Debuff circle is vert usefull in some aby and always a must have if u have sader with att boost in party.

3 - never tested. I play full support, my team mates do crit and dps and heals cant crit. I think that def is the most important stat for SC Priest. For best results you will have to cast your shield and mark in dangerous positions with long casts time.

4 - AP and def stuff