im playing since the start of 4.1 and I always used a mercenary as a summon necro, so let me tell you what i know so far.

best starting merc is the a3 freeze merc for people who don't need corpses (and even if you do, most likely) because of shields. you can make an extremely tanky mercenary who is also a ranged character so he won't be randomly 1 shotted by melee attacks (mostly, again).

best starting gear for a mercenary is (talking about pre-merc amulet patches):

- focus helm (all res max res conc aura etc)
- guardian angel (purely for max all res cap)
- medusa's gaze (holy freeze + 75 all res dr etc)
- azuwrath (salv aura on eq is great early on for any player)

this setup gets you maxed and stacked res for merc. ofc he dies to the weakest magic spells but thats not a huge concern early on

ill add some more stuff later on