I have 5.9k gearscore in my pvp gear but I can do it with lower. Relentless set pieces (saves almost 200k honor compared to buying furious, for doing a few arenas every week and also the items are better but takes some times with the limited amount of arena points), icc ring, wrathful ring, neck, cloak, bracers, relentless belt and boots are all easy high ilvl items. PVP items are pretty bad in PVE but the crit immunity and health is ok and damage works even though PVE gear gets way better damage.
If you want to farm thrash for gold, then you ofcourse want 10 characters for faster speed and PVE gear would also improve speed but take a lot of time to get. (I basically just said I can do it with 5 so it is not hard, not that 5 is efficient)
I don't think tank + 4 shadow priest have enough tank healing to survive but I am not sure, also depends on the gear of the characters, although I have not played that group but I just can't see it being possible unless you are outside of shadowform but then it is probably better to have 1 dedicated to healing.
I have barely done bgs, I basically only did 1 av and 1 ioc, won both but I am weak in big fights and limited effectiveness on doing objectives so not the best. In WG I get 15-20k (forgot how much it is) honor from most battles (so maybe able to buy 7.5-10 gems from 1 battle with each being worth 80-100g each so that should be 600-1000g on Lordaeron) because horde is usually dominating on Lordaeron, often no damage to keep and towers alive and all workshops capped for a maximum finish bonus. Although some battles is much lower honor and I think on Icecrown the teams are much more even so can probably not get that amount of honor. But this amount of gold for a fun activity is great.
When I farmed gold to trade for coins for my weapons on my icecrown team I did dailies, I think I got 1 weapon every 3 days or something like that, not 100% sure and I might have done dailies on 2 teams, it was several years ago but I am pretty sure it was 1 weapon every 3 days. (this is best done right after gold squish since gold is worth more then)